Auto merge of #118032 - RalfJung:char-u32, r=Mark-Simulacrum

guarantee that char and u32 are ABI-compatible

In https://github.com/rust-lang/rust/pull/116894 we added a guarantee that `char` has the same alignment as `u32`, but there is still one axis where these types could differ: function call ABI. So let's nail that down as well: in a function signature, `char` and `u32` are completely equivalent.

This is a new stable guarantee, so it will need t-lang approval.