The Clang modules implementation breaks enough that libc++ needs an easy way
to enable/disable using modules on the Zorg builders. Editing Zorg itself
requires a buildmaster restart which only happens weekly. This patch
allows LIBCXX_USE_MODULES to be used to enable/disable the feature,
allowing the buildslave to disable it as need be.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@288736 91177308-0d34-0410-b5e6-96231b3b80d8