[clang] [clang][ExprConst] Add diagnostics for invalid binary arithmetic (PR #118475)

[Richard Smith via cfe-commits]

https://github.com/zygoloid commented:

Thanks! I think this is an improvement as-is, but while we're looking at this it'd be useful to make the existing diagnostics (and the new ones that are mirroring them) a little bit more precise about what the problem is.

https://github.com/llvm/llvm-project/pull/118475