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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > I'm confused -- how is there existing code depending on a builtin which doesn't yet exist in Clang? Are we copying these builtins from somewhere else?
> 
> This is part of the pointer authentication support we are upstreaming, all of which has been in use for a long time, just not available in upstream llvm/clang.

Oof. On the one hand, we don't want to break your existing users. On the other hand, we don't want to force the much broader audience of users into a worse interface. Are those uses in your downstream able to use things like fix-its to switch from the reference-based interface to a pointer-based interface? (Basically, is there a transition approach that leads to a better world?)

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