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
This worked in standalone build by creating a directory that never got used but it failed in a non-standalone build.
Fix tracked with Dmitri's help.
Swift SVN r24180
