[OBJC] Allow __attribute__((NSObject)) types be used as lightweight generic specifiers (#84593)

As per
https://clang.llvm.org/docs/AutomaticReferenceCounting.html#retainable-object-pointers,
types with `__attribute__((NSObject))` are retainable, and thus should
be eligible to be used as lightweight generic specifiers.

Fix for #84592 84592