[Swiftify] Enable warnings-as-errors in interop test cases (NFC)
This enables -strict-memory-safety -warnings-as-errors on the Swift side
to verify that the macro expansions don't cause any warnings and that
they use `unsafe` correctly. On the clang side it enables -Xcc -Werror.
To reduce noise in the test output and pass -Werror cleanly it also
enables -Xcc -Wno-nullability-completeness. This will make it easier to
detect mistakes when writing tests, because warnings stand out whereas
previously they've been drowned out in the noise.
