[clang] [clang][Sema] Cleanup and optimize DiagnoseAssignmentEnum (PR #141471)

[Timm Baeder via cfe-commits]

tbaederr wrote:

http://llvm-compile-time-tracker.com/compare.php?from=843e362318e884991e517a54446b4faeacdad789&to=7b0a2b83146623a1674967ff4353a9eea6efd25e&stat=instructions:u

https://github.com/llvm/llvm-project/pull/141471