[clang-tools-extra] [clang-tidy][NFC] Run tests in multiple language modes where possible (PR #184741)

[via cfe-commits]

zeyi2 wrote:

>  the script that generates the check skeletton

`add_new_check.py` already uses the `-or-later` form by default.

> Should this be added to the documentation about writing clang-tidy checks...

Thanks for the suggestion! I'm updating the documentation now.

> If I write -std=c++14 the test runner should turn that into c++14-or-later, and only if I really want only one standard, I should write -std=c++14-only.

IMO this might need more discussion (As I suspect this might break backward compatibility in the existing tests?) Maybe we can update the documentation first then decide whether to change the default behaviour later :)

https://github.com/llvm/llvm-project/pull/184741