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

[John McCall via cfe-commits]

rjmccall wrote:

It is very common for type traits to factor into the ABI of a template specialization, such that `my_variant<T>` has a different layout depending on `is_amazing_v<T>`. That is why changing the value of a type trait generally has to be considered an ABI break.

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