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

[via llvm-commits]

https://github.com/cor3ntin approved this pull request.

Ideally we would use constraints for that, when we switch to c++20.
In the meantime it is an improvement !

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