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 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`).
