[clang-tidy] Use a python script instead of a shell script to run clang-tidy tests.

Summary:
Add check_clang_tidy.py script that is functionally identical to the
check_clang_tidy.py, but should also be functional on windows.

I've verified that the script works on linux. Would be nice if folks using
Windows could test the patch before I break windows bots ;)

Reviewers: chapuni, aaron.ballman

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D12180

llvm-svn: 245583