[clang] [clang] Add builtin_get_vtable_pointer and virtual_member_address (PR #135469)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

I just noticed there's no documentation for `__builtin_virtual_member_address()` so I'm addressing that.

Due to code drift and time I realized that they disagree as to whether they should take `C*` or `C&` so I'm going to make them both accept either (existing code means we can't unify on one or the other, but also there's no real reason to not accept either and simply do the correct thing)

https://github.com/llvm/llvm-project/pull/135469