Use built-in model compiler instead of relying on Xcode

[atomicbird]

This updates mogenerator to use an internal model compilation scheme based on momcom whenever the model uses the Xcode 4+ file format. Model files are compiled directly in mogenerator instead of calling out to Xcode's compiler. This removes the dependency on Xcode's shifting location of the momc tool for those files.

Tests in mogenerator/tests pass.

[rentzsch]

Nice! I'm about to ship 1.28, so I'll assign this to 1.29.

[justin]

Is this still relevant @atomicbird? What do we need to do to get it merged in for 1.29?

[atomicbird]

Still relevant, the goal is to have mogenerator handle compiling the model itself instead of figuring out how to call Xcode in a fragile if/then/else. I can try to update this to target the swift12 branch if it's convenient, but it's not really a Swift related change.

[justin]

Thanks, @atomicbird!