[clang] [clang] Diagnose invalid/unavailable operator overloads for scoped flag_enums (PR #218290)

[Corentin Jabot via cfe-commits]

https://github.com/cor3ntin commented:

So this thing will warn for overloads with cvref qualifiers.
I don't know if we care but it's worth considering.

I'm not super familiar with scoped_flags, so I have limited opinions on this PR.

https://github.com/llvm/llvm-project/pull/218290