Rather than forming a set intersection, only look for a common prefix
with the parent arguments. This is both quicker and more correct
since arguments can be order-dependent. This does mean we'll end up
with more per-file arguments for "umbrella" targets, but that at
least will no longer prevent us from forming buildable folders for
them.
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 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`).
