Auto merge of #34141 - eddyb:trans-abi-memcpy, r=nikomatsakis

Auto merge of #34141 - eddyb:trans-abi-memcpy, r=nikomatsakis

trans: always use a memcpy for ABI argument/return casts.

When storing incoming arguments or values returned by call/invoke, always do a `memcpy` from a temporary of the cast type, if there is an ABI cast.
While Clang has gotten smarter ([store](https://godbolt.org/g/EphFuK) vs [memcpy](https://godbolt.org/g/5dikH9)), a `memcpy` will always work.
This is what @dotdash has wanted to do all along, and it fixes #32049.