[xcodegen] Allow buildable folders in more cases
We can define exceptions to handle targets with sources that either
have unique arguments or are unbuildable. Eventually this ought to
allow us to ditch the "no outside-target source file" rule, but I'm
leaving that be for now since ideally we'd handle automatically
splitting up umbrella Clang targets such as `stdlib` such that e.g
`swiftCore` is its own buildable folder instead of an exception.
