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

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> 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?)

I sent this in discord rather than in GH where the reply can be seen by others and not lost to time:

> i'm going to revert the addition of reference parameter i added to __builtin_get_vtable_pointer, for the suboptimal behavior of __builtin_virtual_member_address, i could make it pointer argument only by default, with a flag to permit reference parameters and a deprecation warning so that it's only no new users of a reference argument arrive and we can drop it entirely once all existing users have migrated


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