[clang] [llvm] [clang] Add scoped enum support to `StreamingDiagnostic` (PR #138089)

[Erich Keane via cfe-commits]

erichkeane wrote:

Also, that build error is problematic and seems correct.  The default argument of `is_scoped_enum_v` is depending on `underlying_type_t` which is invalid for non-enums?  Note that templates don't shortcut with instantiation, so you probably have to 'hide' this somewhere as well.

https://github.com/llvm/llvm-project/pull/138089