Fix Access Violation when using lld & ThinLTO on windows-msvc

Users report an AV at runtime of the compiled binary when using lld and
ThinLTO on windows-msvc. The AV occurs when accessing a static value
which is defined in one crate but used in another. Based on the
disassembly of the cross-crate use, it appears that the use is not
correctly linked with the definition and is instead assigned a garbage
pointer value.

If we look at the symbol tables for each crates' obj file, we can see
what is happening:

*lib.obj*:

```
COFF SYMBOL TABLE
...
00E 00000000 SECT2  notype       External     | _ZN10reproducer7memrchr2FN17h612b61ca0e168901E
...
```

*bin.obj*:

```
COFF SYMBOL TABLE
...
010 00000000 UNDEF  notype       External     | __imp__ZN10reproducer7memrchr2FN17h612b61ca0e168901E
...
```

The use of the symbol has the "import" style symbol name but the
declaration doesn't generate any symbol with the same name. As a result,
linking the files generates a warning from lld:

> rust-lld: warning: bin.obj: locally defined symbol imported: reproducer::memrchr::FN::h612b61ca0e168901 (defined in lib.obj) [LNK4217]

and the symbol reference remains undefined at runtime leading to the AV.

To fix this, we just need to detect that we are performing ThinLTO (and
thus, static linking) and omit the `dllimport` attribute on the extern
item in LLVM IR.

compiler/rustc_codegen_llvm/src/consts.rs

@@ -19,6 +19,7 @@ use rustc_middle::mir::mono::MonoItem;
 use rustc_middle::ty::layout::LayoutOf;
 use rustc_middle::ty::{self, Instance, Ty};
 use rustc_middle::{bug, span_bug};
+use rustc_session::config::Lto;
 use rustc_target::abi::{
     AddressSpace, Align, HasDataLayout, Primitive, Scalar, Size, WrappingRange,
 };
@@ -303,7 +304,8 @@ impl<'ll> CodegenCx<'ll, '_> {
                 // ThinLTO can't handle this workaround in all cases, so we don't
                 // emit the attrs. Instead we make them unnecessary by disallowing
                 // dynamic linking when linker plugin based LTO is enabled.
-                !self.tcx.sess.opts.cg.linker_plugin_lto.enabled();
+                !self.tcx.sess.opts.cg.linker_plugin_lto.enabled() &&
+                self.tcx.sess.lto() != Lto::Thin;
 
             // If this assertion triggers, there's something wrong with commandline
             // argument validation.

src/test/codegen/issue-81408-dllimport-thinlto-windows.rs

@@ -8,7 +8,7 @@
 extern crate static_dllimport_aux;
 
 // CHECK-LABEL: @{{.+}}CROSS_CRATE_STATIC_ITEM{{.+}} =
-// CHECK-SAME: external dllimport local_unnamed_addr global %"{{.+}}::AtomicPtr
+// CHECK-SAME: external local_unnamed_addr global %"{{.+}}AtomicPtr
 
 pub fn main() {
     static_dllimport_aux::memrchr();