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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

Apparently, my implementation of `is_scoped_enum_v` is not SFINAE-able enough. I'm working on a fix.

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