compiler: analyze type and value of global declaration separately

This commit separates semantic analysis of the annotated type vs value
of a global declaration, therefore allowing recursive and mutually
recursive values to be declared.

Every `Nav` which undergoes analysis now has *two* corresponding
`AnalUnit`s: `.{ .nav_val = n }` and `.{ .nav_ty = n }`. The `nav_val`
unit is responsible for *fully resolving* the `Nav`: determining its
value, linksection, addrspace, etc. The `nav_ty` unit, on the other
hand, resolves only the information necessary to construct a *pointer*
to the `Nav`: its type, addrspace, etc. (It does also analyze its
linksection, but that could be moved to `nav_val` I think; it doesn't
make any difference).

Analyzing a `nav_ty` for a declaration with no type annotation will just
mark a dependency on the `nav_val`, analyze it, and finish. Conversely,
analyzing a `nav_val` for a declaration *with* a type annotation will
first mark a dependency on the `nav_ty` and analyze it, using this as
the result type when evaluating the value body.

The `nav_val` and `nav_ty` units always have references to one another:
so, if a `Nav`'s type is referenced, its value implicitly is too, and
vice versa. However, these dependencies are trivial, so, to save memory,
are only known implicitly by logic in `resolveReferences`.

In general, analyzing ZIR `decl_val` will only analyze `nav_ty` of the
corresponding `Nav`. There are two exceptions to this. If the
declaration is an `extern` declaration, then we immediately ensure the
`Nav` value is resolved (which doesn't actually require any more
analysis, since such a declaration has no value body anyway).
Additionally, if the resolved type has type tag `.@"fn"`, we again
immediately resolve the `Nav` value. The latter restriction is in place
for two reasons:

* Functions are special, in that their externs are allowed to trivially
  alias; i.e. with a declaration `extern fn foo(...)`, you can write
  `const bar = foo;`. This is not allowed for non-function externs, and
  it means that function types are the only place where it is possible
  for a declaration `Nav` to have a `.@"extern"` value without actually
  being declared `extern`. We need to identify this situation
  immediately so that the `decl_ref` can create a pointer to the *real*
  extern `Nav`, not this alias.
* In certain situations, such as taking a pointer to a `Nav`, Sema needs
  to queue analysis of a runtime function if the value is a function. To
  do this, the function value needs to be known, so we need to resolve
  the value immediately upon `&foo` where `foo` is a function.

This restriction is simple to codify into the eventual language
specification, and doesn't limit the utility of this feature in
practice.

A consequence of this commit is that codegen and linking logic needs to
be more careful when looking at `Nav`s. In general:

* When `updateNav` or `updateFunc` is called, it is safe to assume that
  the `Nav` being updated (the owner `Nav` for `updateFunc`) is fully
  resolved.
* Any `Nav` whose value is/will be an `@"extern"` or a function is fully
  resolved; see `Nav.getExtern` for a helper for a common case here.
* Any other `Nav` may only have its type resolved.

This didn't seem to be too tricky to satisfy in any of the existing
codegen/linker backends.

Resolves: #131

src/Compilation.zig

@@ -2906,6 +2906,7 @@ const Header = extern struct {
         file_deps_len: u32,
         src_hash_deps_len: u32,
         nav_val_deps_len: u32,
+        nav_ty_deps_len: u32,
         namespace_deps_len: u32,
         namespace_name_deps_len: u32,
         first_dependency_len: u32,
@@ -2949,6 +2950,7 @@ pub fn saveState(comp: *Compilation) !void {
                 .file_deps_len = @intCast(ip.file_deps.count()),
                 .src_hash_deps_len = @intCast(ip.src_hash_deps.count()),
                 .nav_val_deps_len = @intCast(ip.nav_val_deps.count()),
+                .nav_ty_deps_len = @intCast(ip.nav_ty_deps.count()),
                 .namespace_deps_len = @intCast(ip.namespace_deps.count()),
                 .namespace_name_deps_len = @intCast(ip.namespace_name_deps.count()),
                 .first_dependency_len = @intCast(ip.first_dependency.count()),
@@ -2979,6 +2981,8 @@ pub fn saveState(comp: *Compilation) !void {
         addBuf(&bufs, mem.sliceAsBytes(ip.src_hash_deps.values()));
         addBuf(&bufs, mem.sliceAsBytes(ip.nav_val_deps.keys()));
         addBuf(&bufs, mem.sliceAsBytes(ip.nav_val_deps.values()));
+        addBuf(&bufs, mem.sliceAsBytes(ip.nav_ty_deps.keys()));
+        addBuf(&bufs, mem.sliceAsBytes(ip.nav_ty_deps.values()));
         addBuf(&bufs, mem.sliceAsBytes(ip.namespace_deps.keys()));
         addBuf(&bufs, mem.sliceAsBytes(ip.namespace_deps.values()));
         addBuf(&bufs, mem.sliceAsBytes(ip.namespace_name_deps.keys()));
@@ -3145,7 +3149,7 @@ pub fn getAllErrorsAlloc(comp: *Compilation) !ErrorBundle {
 
             const file_index = switch (anal_unit.unwrap()) {
                 .@"comptime" => |cu| ip.getComptimeUnit(cu).zir_index.resolveFile(ip),
-                .nav_val => |nav| ip.getNav(nav).analysis.?.zir_index.resolveFile(ip),
+                .nav_val, .nav_ty => |nav| ip.getNav(nav).analysis.?.zir_index.resolveFile(ip),
                 .type => |ty| Type.fromInterned(ty).typeDeclInst(zcu).?.resolveFile(ip),
                 .func => |ip_index| zcu.funcInfo(ip_index).zir_body_inst.resolveFile(ip),
             };
@@ -3380,7 +3384,7 @@ pub fn addModuleErrorMsg(
                 defer gpa.free(rt_file_path);
                 const name = switch (ref.referencer.unwrap()) {
                     .@"comptime" => "comptime",
-                    .nav_val => |nav| ip.getNav(nav).name.toSlice(ip),
+                    .nav_val, .nav_ty => |nav| ip.getNav(nav).name.toSlice(ip),
                     .type => |ty| Type.fromInterned(ty).containerTypeName(ip).toSlice(ip),
                     .func => |f| ip.getNav(zcu.funcInfo(f).owner_nav).name.toSlice(ip),
                 };
@@ -3647,6 +3651,7 @@ fn performAllTheWorkInner(
                 try comp.queueJob(switch (outdated.unwrap()) {
                     .func => |f| .{ .analyze_func = f },
                     .@"comptime",
+                    .nav_ty,
                     .nav_val,
                     .type,
                     => .{ .analyze_comptime_unit = outdated },
@@ -3679,7 +3684,7 @@ fn processOneJob(tid: usize, comp: *Compilation, job: Job, prog_node: std.Progre
                     return;
                 }
             }
-            assert(nav.status == .resolved);
+            assert(nav.status == .fully_resolved);
             comp.dispatchCodegenTask(tid, .{ .codegen_nav = nav_index });
         },
         .codegen_func => |func| {
@@ -3709,6 +3714,7 @@ fn processOneJob(tid: usize, comp: *Compilation, job: Job, prog_node: std.Progre
 
             const maybe_err: Zcu.SemaError!void = switch (unit.unwrap()) {
                 .@"comptime" => |cu| pt.ensureComptimeUnitUpToDate(cu),
+                .nav_ty => |nav| pt.ensureNavTypeUpToDate(nav),
                 .nav_val => |nav| pt.ensureNavValUpToDate(nav),
                 .type => |ty| if (pt.ensureTypeUpToDate(ty)) |_| {} else |err| err,
                 .func => unreachable,
@@ -3734,7 +3740,7 @@ fn processOneJob(tid: usize, comp: *Compilation, job: Job, prog_node: std.Progre
                 // Tests are always emitted in test binaries. The decl_refs are created by
                 // Zcu.populateTestFunctions, but this will not queue body analysis, so do
                 // that now.
-                try pt.zcu.ensureFuncBodyAnalysisQueued(ip.getNav(nav).status.resolved.val);
+                try pt.zcu.ensureFuncBodyAnalysisQueued(ip.getNav(nav).status.fully_resolved.val);
             }
         },
         .resolve_type_fully => |ty| {