Add PLATFORM_DIR support

[shepting]

We have a few Xcode project files with the value of PLATFORM_DIR where most projects have set SOURCE_ROOT.

This most often occurs in something like this:

1DD70E00000 /* XCTest.framework */ = {isa = PBXFileReference; explicitFileType = wrapper.framework; name = XCTest.framework; path = Developer/Library/Frameworks/XCTest.framework; sourceTree = PLATFORM_DIR; };

[shepting]

@tomlokhorst Any chance we could get this in?

[tomlokhorst]

Yes, this looks good. Thanks for the PR!