Auto merge of #115372 - RalfJung:abi-assert-eq, r=davidtwco

add rustc_abi(assert_eq) to test some guaranteed or at least highly expected ABI compatibility guarantees

This new repr(transparent) test is super useful, it would have found rust-lang/rust#115336 and found rust-lang/rust#115404, rust-lang/rust#115481, rust-lang/rust#115509.