Issues with CMake support

~I have a suspicion that [this bug](https://github.com/dabrahams/cmake-xcode-module-path-bug) may be triggered by the CMakeLists doing things they probably shouldn't, e.g. globally setting variables like `CMAKE_Swift_MODULE_DIRECTORY` even when it is not top-level and then using that to set `INTERFACE_INCLUDE_DIRECTORIES` properties. Obviously, the CMake Xcode generator “should” be resilient to any arbitrary setup you choose, but it's probably impossible for its author to avoid making some assumptions of regularity. I know what patch to offer for those top level variables, but then the `INTERFACE_INCLUDE_DIRECTORIES` settings stop having their intended effect.~

I'd like to know what cmake + Ninja commands are expected to work on this project so that I can propose changes that preserve that functionality.

/cc @compnerd who wrote most of the CMake support.

Start by identifying which CMake and Ninja command combinations are expected to work for this project, then inspect the CMakeLists settings for CMAKE_Swift_MODULE_DIRECTORY and INTERFACE_INCLUDE_DIRECTORIES. Done means the supported commands and their expected behavior are clear, with proposed changes preserving that functionality.

Written by the indexing model from the issue text.