Since we now have modules-enabled CI, it is now redundant to have ad-hoc
tests that check arbitrary things about our modules support. Instead,
the whole test suite should pass with modules enabled, period.
This patch also removes the module cache path workaround: one would
expect that modules work properly without that workaround. If that
isn't the case and we do run into flaky test failures, we can re-enable
the workaround temporarily (but that would be very vexing and we should
fix Clang ASAP if that's the case).
Differential Revision: https://reviews.llvm.org/D104746
0c0628c92c