Invoke the linker if there are any -Xlinker arguments, even if there are no inputs

Sometimes it is useful to invoke the linker via swiftc (because of its extra behaviors), but needing to use -Xlinker to escape things so swiftc doesn’t muck with them. The driver should be willing to invoke the linker even when there are no input files if there are some Xlinker arguments.

The clang driver has a similar special case for linker args.