Test @_objcImpl stored property access semantics

Accessing a stored property from a normal method should send a selector, but accessing it from an initializer or accessor should use the field offset.

Author: beccadax

test/IRGen/Inputs/objc_implementation.h

@@ -2,6 +2,8 @@
 
 @interface ImplClass: NSObject
 
+- (nonnull instancetype)init;
+
 @property (assign) int implProperty;
 
 - (void)mainMethod:(int)param;