Objective-C : Implement NSObject attribute.

Objective-C : Implement NSObject attribute.

This attribute allows pointers to be marked as pointers to
an NSObject-compatible object.  This allows for additional
checking of assignment etc. when refering to pointers to
opaque types.

gcc/c-family/ChangeLog:

	* c-attribs.c (handle_nsobject_attribute): New.
	* c.opt: Add WNSObject-attribute.

gcc/objc/ChangeLog:

	* objc-act.c (objc_compare_types): Handle NSObject type
	attributes.
	(objc_type_valid_for_messaging): Likewise.

gcc/testsuite/ChangeLog:

	* obj-c++.dg/attributes/nsobject-01.mm: New test.
	* objc.dg/attributes/nsobject-01.m: New test.