[clang] adds `__is_scoped_enum`, `__is_nullptr`, and `__is_referenceable`

... as builtins.

This is information that the compiler already has, and should be exposed
so that the library doesn't need to reimplement the exact same
functionality.

This was originally a part of D116280.

Depends on D135175.

Differential Revision: https://reviews.llvm.org/D135177