Use -Xlinker when constructing response files for Swift AST paths

[owenv]

This way we don't need to vary the args based on which compiler is the linker driver

rdar://148630017

[owenv]

@swift-ci test

[mhrawdon]

Yes, -Wl, is especially bad if the path being passed itself contains a comma (the tool often fails to parse the argument). We switched to using -Xlinker for this reason some time ago, and I recommend to everyone that they do the same in their projects. But it's an inertia which is hard to overcome.