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

[Mark de Wever via cfe-commits]

mordante wrote:

> > Should we prefix this with `__builtin_` as mentioned in [#98310 (comment)](https://github.com/llvm/llvm-project/issues/98310#issuecomment-2221105713) ?
> 
> Yes, if we are doing this, we should be consistent going forward

I would like that. It's easier directly determine `__builtin_foo` is a builtin instead of just  `__foo`. Especially in the standard library we use a lot of `__ugly_names` .

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