Build the Apple-style stage2 with modules

Green dragon had a green stage2 modules bot for a long time now[1] and
it is time to retire it and make a modules build the default for
Apple-style stage2 builds.

This patch turns on LLVM_ENABLE_MODULES.

[1] http://green.lab.llvm.org/green/job/clang-stage2-cmake-modulesRDA_build/
rdar://problem/28672159

Differential Revision: https://reviews.llvm.org/D32603

llvm-svn: 302556