[clang-tidy] Parallelize clang-tidy-diff.py
[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



git-svn-id: a34e9779ed74578ad5922b3306b3d80a0c825546@356547 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang-tools-extra,llvm-mirror/clang-tools-extra,llvm-mirror/clang-tools-extra,llvm-mirror/clang-tools-extra