[clangd] Fix check-clangd with no clang built

Authored by sammccall on Oct 31 2020, 4:36 AM.


[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