[clang] [clang][ExprConst] Add `EvalStatus::DiagEmitted` flag (PR #203838)

[Timm Baeder via cfe-commits]

tbaederr wrote:

https://llvm-compile-time-tracker.com/compare.php?from=05c8f9b1ead844024f414f45ab9ba11757bef2ea&to=7506733f41a8ecc38408440e826b957c341d7789&stat=instructions:u

https://github.com/llvm/llvm-project/pull/203838