Detect if an .xcdatamodeld is not being compiled by any targets

[kieranjlynch]

Where a CoreData model is situated in something like a Framework, it would be useful to detect if it wasn't present in "Compile Sources" in the main app target

[mattmassicotte]

I'm pretty sure that XCLint can iterate over all of the referenced files and complain if they are not referenced by any target. Seems like a good addition.