[libc++] Remove ad-hoc modules tests that are now unnecessary

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

Author: ldionne

libcxx/test/libcxx/modules/cinttypes_exports.compile.pass.cpp

@@ -43,15 +43,6 @@
                                                                  sys.platform.lower().strip() == 'darwin'), # TODO: this doesn't handle cross-compiling to Apple platforms.
   Feature(name='objective-c++',                 when=lambda cfg: hasCompileFlag(cfg, '-xobjective-c++ -fobjc-arc')),
 
-  # Note: We use a custom modules cache path to make sure that we don't reuse
-  #       the default one, which can be shared across builds. This is important
-  #       because we define macros in headers files, and a change in these macros
-  #       doesn't seem to invalidate modules cache entries, which means we could
-  #       build against now-invalid cached headers from a previous build.
-  Feature(name='modules-support',
-          when=lambda cfg: hasCompileFlag(cfg, '-fmodules'),
-          actions=lambda cfg: [AddCompileFlag('-fmodules-cache-path=%t/ModuleCache')]),
-
   Feature(name='non-lockfree-atomics',
           when=lambda cfg: sourceBuilds(cfg, """
             #include <atomic>