[clang-tidy] Fix file extension inconsistency

[googlewalt]

This fixes an issue with #150791. In CheckRunner, we treat files with unrecognized extensions as ".cpp", by forcefully assigning extension = ".cpp" if it's not already one of ".c", ".hpp", ".m", or ".mm". Make the new code which chooses the default -std argument be consistent with that, so that using other file extensions doesn't trigger an error message like error: invalid argument '-std=c99' not allowed with 'C++'

[github-actions]

✅ With the latest revision this PR passed the Python code formatter.

[nicovank]

Assuming you're talking about this default?

llvm-project/clang-tools-extra/test/clang-tidy/check_clang_tidy.py

Lines 113 to 114 in 3c7bf3b

	if extension not in [".c", ".hpp", ".m", ".mm"]:
	extension = ".cpp"

Seems fine to me. I'll defer to @localspook.

[googlewalt]

Assuming you're talking about this default?

Yes. This addresses some internal tests breakages.

[jyknight]

Given that the new change produces clearly incorrect behavior currently, I'm gonna approve+merge this.