- pass required=False to use_clang(), as we don't need it
- fix required=False (which was unused and rotted):
- make derived substitutions conditional on it
- add a feature so we can disable tests that need it
- conditionally disable our one test that depends on %resource_dir.
This doesn't seem right from first principles, but isn't a big deal.
Differential Revision: https://reviews.llvm.org/D90528
