[clangd] Fix check-clangd with no clang built

- 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