Use the virtual name of headers when searching for a module

When using the VFS, we want the virtual header location when searching
for a framework module, since that will be the one in the correct
directory structure for the module.

I'll add a regression test once I finish reducing the larger one I have.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@208901 91177308-0d34-0410-b5e6-96231b3b80d8