RFC: Removing Objective-C Garbage Collection from Clang

It has been over three years since Apple Clang has supported -fobjc-gc (Objective-C garbage collection, which predates before -fobjc-arc). The code is still there in Clang, but my understanding is that no one is using it.

I'd like to clean things up and delete the code.

Please let me know if there are any objections! Otherwise I'm planning to start on this sometime next week.

Today is Monday, but I'm happy to wait a little longer (not going to do it today in any case). Would you like me to hold off until next week?

I don't see anything worth holding off for. Thanks for giving me time to review.