[MNT]: macOS backend should always pair +alloc/-init calls #31798

Summary

I mentioned the following in #21788:

In Objective-C, an -init… method should always be called immediately after +alloc. Calling any other instance method on an object you just allocated before calling -init… is undefined behavior.¹²³

I believe that each tp_new callback should only call tp_alloc and not instantiate an uninitialized Objective-C object. Each tp_init callback should then create a fully initialized Objective-C object via an +alloc/-init… pair.

I'm starting work on #31797 and the use of unpaired +alloc/-init… calls is generating a few warnings. I'd like to clean this up at the same time I do the ARC work.

Proposed fix

No response