[clang] [clang][PAC] Support trivially_relocating polymorphic objects (PR #144420)

[John McCall via cfe-commits]

rjmccall wrote:

I've been informed that I've misunderstood the nature of "trivial relocation" — it is not a "trivial" operation in the usual sense of triviality, it's just an operation that (1) can be done primitively by the compiler and (2) cannot fail. That addresses my concern, because we can certainly implement that for polymorphic classes.

I think we should make a point of implementing this for address-discriminated `__ptrauth` qualifiers before we release it, because changing the type trait in a future release will be an ABI break.

https://github.com/llvm/llvm-project/pull/144420