Reland r356547 after fixing the tests for Linux.
[clang-tidy] Parallelize clang-tidy-diff.py
This patch has 2 rationales:
- large patches lead to long command lines and often cause max command line length restrictions imposed by OS;
- clang-tidy runs on modified files are independent and can be done in parallel, the same as done for run-clang-tidy.
Differential Revision: https://reviews.llvm.org/D57662