Issue Description:

In Swift, the following is permitted:

protocol Foo {
    static func makeFoo() -> Self
}

But if the protocol is marked as @objc, it generates the warning "Method cannot be a member of an @objc protocol because its result type cannot be represented in Objective-C"

However, adding Self as a return type to a protocol method doesn't usually treat Self as an associatedType requirement, so it's not clear why a protocol method returning Self can't be represented in Objective-C.

The equivalent Objective-C code is permitted and works as you'd expect:

@protocol Foo
+ (instancetype)makeFoo;
@end

@interface Bar: NSObject<Foo>
@end

@implementation Bar
+ (instancetype)makeFoo { return [[self alloc] init]; }
@end