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

[Baranov Victor via cfe-commits]

vbvictor wrote:

Note about [comment](https://github.com/llvm/llvm-project/issues/148624#issuecomment-3069680087) by Eugene:

`run-clang-tidy` already tried to run on max cpu cores by default, so problem with LFS was here for 11 years since birth of the file in https://reviews.llvm.org/D5188.

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