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 https://github.com/rust-lang/rust/pull/39999#issuecomment-287723379).

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.