[clang] [clang][Sema] Optimize ~SemaDiagnosticBuilder (PR #142252)

[Timm Baeder via cfe-commits]

tbaederr wrote:

The original code looks like the ordering with the `ImmediateDiag.reset()` call matters, but it doesn't seem like it does.

https://github.com/llvm/llvm-project/pull/142252