Rollup merge of #55834 - ogoffart:union-abi, r=eddyb

Forward the ABI of the non-zero sized fields of an union if they have the same ABI

This is supposed to fix the performence regression of using MaybeUninit in
https://github.com/rust-lang/rust/pull/54668

diff --cc src/librustc/ty/layout.rs
Simple merge

diff --cc src/librustc/ty/mod.rs
Simple merge