Xcode doesn't appear to like use of C++20 and -fcxx-modules

[alecazam]

I've had to revert a whole bunch of clang module use in order to enable C++20 support in Xcode. I had to turn off GLTFKit as a result, but will see about adding it back with regular includes.

None of my Objective-C++ files would compile once C++17 was changed to C++20.