[clang][modules] Don't depend on sharing FileManager during module build

Sharing the FileManager between the importer and the module build should
only be an optimization. Add a cc1 option -fno-modules-share-filemanager
to allow us to test this. Fix the path to modulemap files, which
previously depended on the shared FileManager when using path mapped to
an external file in a VFS.

Differential Revision: https://reviews.llvm.org/D131076