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

[via cfe-commits]

https://github.com/Sirraide commented:

Overall, the implementation seems fine. 

Do we want a release note for this since it’s supposed to be an internal thing? 
CC @AaronBallman 

Also, some documentation *somewhere* as to what this builtin does would be nice, even if it’s just in a comment somewhere. It took me a bit to figure out what exactly its semantics are supposed to be.

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