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.
The intention here was to do a component-wise
prefix check, not sure why I did a string prefix
check. Switch to component prefix and rename to
`starts(with:)` to match `FilePath`.
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`).
