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

[Christopher Di Bella via cfe-commits]

cjdb wrote:

I spent a lot of time trying to implement this myself, but I always struggled to capture detecting user specialisations of `common_type`. Which section of code handles that?

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