Make module files passed to a module build via -fmodule-file= available to

Make module files passed to a module build via -fmodule-file= available to
consumers of that module.

Previously, such a file would only be available if the module happened to
actually import something from that module.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@232583 91177308-0d34-0410-b5e6-96231b3b80d8