[libc++] Deflake the Clang Modules CI job

This re-introduces the workaround that had been introduced in d7ca140
and then removed in 0c0628c, since it seems like it is needed after all.

Differential Revision: https://reviews.llvm.org/D157319

Author: ldionne

libcxx/utils/libcxx/test/params.py

@@ -127,6 +127,10 @@ def getModuleFlag(cfg, enable_modules):
             AddFeature("modules-build"),
             AddCompileFlag("-fmodules"),
             AddCompileFlag("-fcxx-modules"), # AppleClang disregards -fmodules entirely when compiling C++. This enables modules for C++.
+            # Note: We use a custom modules cache path to make sure that we don't reuse
+            #       the default one, which can be shared across CI builds with different
+            #       configurations.
+            AddCompileFlag(lambda cfg: f"-fmodules-cache-path={cfg.test_exec_root}/ModuleCache"),
         ]
         if enable_modules == "clang"
         else [