Auto merge of #40658 - eddyb:lay-more-out, r=arielb1

Use ty::layout for ABI computation instead of LLVM types.

This is the first step in creating a backend-agnostic library for computing call ABI details from signatures.
I wanted to open the PR *before* attempting to move `cabi_*` from trans to avoid rebase churn in #39999.
**EDIT**: As I suspected, #39999 needs this PR to fully work (see #39999 (comment)).

The first 3 commits add more APIs to `ty::layout` and replace non-ABI uses of `sizing_type_of`.
These APIs are probably usable by other backends, and miri too (cc @stoklund @solson).

The last commit rewrites `rustc_trans::cabi_*` to use `ty::layout` and new `rustc_trans::abi` APIs.
Also, during the process, a couple trivial bugs were identified and fixed:
* `msp430`, `nvptx`, `nvptx64`: type sizes *in bytes* were compared with `32` and `64`
* `x86` (`fastcall`): `f64` was incorrectly not treated the same way as `f32`

Although not urgent, this PR also uses the more general "homogenous aggregate" logic to fix #32045.

src/librustc/middle/intrinsicck.rs

@@ -89,7 +89,7 @@ impl<'a, 'gcx, 'tcx> ExprVisitor<'a, 'gcx, 'tcx> {
             let from = unpack_option_like(self.infcx.tcx.global_tcx(), from);
             match (&from.sty, sk_to) {
                 (&ty::TyFnDef(..), SizeSkeleton::Known(size_to))
-                        if size_to == Pointer.size(&self.infcx.tcx.data_layout) => {
+                        if size_to == Pointer.size(self.infcx) => {
                     struct_span_err!(self.infcx.tcx.sess, span, E0591,
                                      "`{}` is zero-sized and can't be transmuted to `{}`",
                                      from, to)