[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

llvm-svn: 356547