swift

517ced4f - [ClangImporter][Tests] clang doesn't support the empty string as a path argument

[ClangImporter][Tests] clang doesn't support the empty string as a path argument SIL/verify_all_overlays.py is passing `-F ""` on non-Apple platforms. Swift handles that kind of, but clang doesn't support it and will get argument parsing errors. It's a pathological case, so fix SIL/verify_all_overlays.py to not do that, but also add a failsafe in ClangImporter to not pass on empty paths. rdar://142441042