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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

Should we prefix this with `__builtin_` as mentioned in https://github.com/llvm/llvm-project/issues/98310#issuecomment-2221105713 ?

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