[clang-tools-extra] [clang-tidy] Add parallel execution by default in 'run-clang-tidy' and 'clang-tidy-diff' (PR #149739)

[Carlos Galvez via cfe-commits]

https://github.com/carlosgalvezp approved this pull request.

LGTM!

Not sure if possible but a less aggressive option would be to have a "-l" flag like "make" has, which uses as many threads as requested while keeping track on the load of the system and adjusting accordingly.

https://github.com/llvm/llvm-project/pull/149739