[clang] [libcxx] [Clang] Add __builtin_common_reference (PR #121199)

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

This seems reasonable to me?  I'm not a huge fan of how much is being put into `SemaTemplate.cpp` to support all the builtins though.  I'm wondering if we need to split that up in a follow-up and have a `SemaTemplateBuiltins` for all of these to live.

Please let Aaron/others take a poke at this, and make sure there are a few more Libc++ reviewers to make sure you're all in agreement with how this works (I see the convo that is happening).

Also, should we loop in libstdc++ folks here?

https://github.com/llvm/llvm-project/pull/121199