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

[Vlad Serebrennikov via llvm-commits]

Endilll wrote:

> Ideally we would use constraints for that, when we switch to c++20.

I'd love to, yeah

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