[CodeGen] Treat ObjC `__unsafe_unretained` and class types as trivial

when generating copy/dispose helper functions

Analyze the block captures just once before generating copy/dispose
block helper functions and honor the inert `__unsafe_unretained`
qualifier. This refactor fixes a bug where captures of ObjC
`__unsafe_unretained` and class types were needlessly retained/released
by the copy/dispose helper functions.

Differential Revision: https://reviews.llvm.org/D116948