[clang] [clang][PAC] Add __builtin_get_vtable_pointer (PR #139790)

[via cfe-commits]

https://github.com/cor3ntin commented:

Can you add tests for const types, and tests that check
`__is_same_as(decltype(__builtin_get_vtable_pointer(foo)), const void*)` ?

LGTM otherwise

https://github.com/llvm/llvm-project/pull/139790