Allow enabling/disabling testing with module using env LIBCXX_USE_MODULES=1

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.

llvm-svn: 288736