[clang/objc] Optimize getters for non-atomic, copied properties

Properties that were declared `@property(copy, nonatomic) id foo` make an
unnecessary call to objc_get_property().  This call can be replaced with a
direct access to the backing variable identical to how a `@property(nonatomic)
id foo` would do it.

This reduces codegen by 4 bytes (x86_64/arm64) and removes a cross linkage unit
function call per property declared as copy/nonatomic.

Differential Revision: https://reviews.llvm.org/D105311