Generate a new 'SwiftRuntimes' xcodeproj if a build directory
for 'Runtimes' is specified with `--runtimes-build-dir`, along with a
combined 'Swift+Runtimes' workspace.
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`).