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

[Oliver Hunt via cfe-commits]

ojhunt wrote:

Closing this, as I'm not sure the virtual_member_address builtin is actually needed, so for now we'll just do `builtin_get_vtable` which is used in other projects

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