This test ensures we can configure and build mixed source binaries. The
test configures, but fails to verify due to multiple targets emitting
the `lib.c.o` and `lib.cpp.o` outputs. Both the clang build step and the
swift link step report that they emit the `lib.c.o` and `lib.cpp.o`
outputs. The `.o`'s are emitted by clang, not by swift.
