[clang] [libcxx] [Clang] Add __common_type builtin (PR #99473)

[Nikolas Klauser via cfe-commits]

https://github.com/philnik777 commented:

> How much of a performance difference are you measuring with this change?

That depends heavily on what you're looking at. For `common_type`s with a lot of builtin types, I've seen a 10x improvement. When it's basically just combining specializations I've seen as low as a 1.25x improvement. When having classes that are convertible to another there is about a 2x improvement.

https://github.com/llvm/llvm-project/pull/99473