This enables the use of folder references for
compatible targets, allowing new source files to
be added without needing to regenerate the project.
Currently disabled by default; I'd like to get
some living-on before enabling.
This requires ensuring we check that we're not
ever attempting to add a group under an existing
folder reference. This will be required for
buildable folders.
This is a tool specifically designed to generate
Xcode projects for the Swift repo (as well as a
couple of adjacent repos such as LLVM and Clang).
It aims to provide a much more user-friendly experience
than the CMake Xcode generation (`build-script --xcode`).
