Ninja: Fix mixed Swift/CXX library target generation

With how things were before, mixed Swift/C++ libraries would result in a
broken ninja file.  `cpp.cpp.o` was emitted by the compiler, but was
also being included in the `linkBuild.Outputs` list, so it was being
emitted by multiple targets.

The fix checks that the source language is Swift before adding it to the
list of additional outputs. If it is Swift, this isn't a problem. If it
isn't Swift, we don't include it in the list of outputs.

On the other side, the C++ file was also being passed as a source file,
which the Swift compiler can't compile. So we add the C++ object file as
an explicit dependency and the object file is then added to the list of
Swift sources.