clang-tidy: Add a basic python script to generate checks.

There are still a vast range of improvements that can be done to this,
but it seems like an ok initial version. Suggestions or patches are
highly welcome.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@223766 91177308-0d34-0410-b5e6-96231b3b80d8