[CodeGen][ObjC] Call synthesized copy constructor/assignment operator

functions in getter/setter functions of non-trivial C struct properties

This fixes a bug where the getter/setter functions were doing a trivial
copy instead of calling the synthesized functions that copy non-trivial
C struct types.

This fixes https://github.com/llvm/llvm-project/issues/56680.

Differential Revision: https://reviews.llvm.org/D131701