Don't load Framework module.map files when searching subdirectories

This would cause frameworks to have spurious "redefinition" errors if
they had both a (legacy) "module.map" and a (new) "module.modulemap" file and we
happened to do a sub-directory search in that directory using a
non-framework include path (e.g. -Ifoo/ -Ffoo/).  For migration
purposes it's very handy that the compiler will prefer the new spelling
of the filename and not look at the old one if it doesn't need to.

llvm-svn: 230308