Revert "Disallow using -fmodules with -no-integrated-as."

Per comment from Benjamin Kramer, this isn't portable (yet).  I'll come up
with a better fix.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176876 91177308-0d34-0410-b5e6-96231b3b80d8