In ARC we emit an error when compiling:

@interface Foo : NSObject
@property (readonly) id myProp;
@end

@implementation Foo
@synthesize myProp;
@end

t.m:9:13: error: ARC forbids synthesizing a property of an Objective-C object with unspecified storage attribute
@synthesize myProp;
            ^

which is fine, we want the ownership of the synthesized ivar to be explicit. But we should _not_ emit an error
for the following cases, because we can get the ownership either from the declared ivar or from the property type:

@interface Foo : NSObject {
	__weak id _myProp1;
	id myProp2;
}
@property (readonly) id myProp1;
@property (readonly) id myProp2;
@property (readonly) __strong id myProp3;
@end

@implementation Foo
@synthesize myProp1 = _myProp1;
@synthesize myProp2;
@synthesize myProp3;
@end

rdar://9844006.

llvm-svn: 136155