swift234

cmake: Add a SWIFT_XCODE_GENERATE_FOR_IDE_ONLY flag.

This flag disables the generation of dependency targets that are necessary to accurately rebuild Swift code, but which completely tank the Xcode IDE experience. Since Xcode is primarily useful as a source editor/navigator for the compiler C++ code, and Ninja is a more performant and featureful build system for Swift at this point, provide a mode that generates enough of an Xcode project to edit the Swift compiler source, but which can't build it, as a compromise so we can use Xcode's editor alongside a Ninja build environment.

Swift SVN r24186