[clang] [libcxx] [Clang] Add __builtin_common_type (PR #99473)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

Changes LGTM! It would be nice to add an additional test case that does -ast-dump (or I suppose -ast-print would work as well) to verify the builtin common type list has the expected form. However, I don't insist as I expect the libc++ usage to quickly tell us if we change the signature inadvertently.

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