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 ;)
$(dirname %s) assumes shell. use %S instead.
Lit internal runner is not capable of !shbang. use %python.