Objective-C/C++ : Handle parsing @property 'class' attribute.

This attribute states that a property is one manipulated by class
methods (it requires a static variable and the setter and getter
must be provided explicitly, they cannot be @synthesized).

gcc/c-family/ChangeLog:

	* c-common.h (OBJC_IS_PATTR_KEYWORD): Add class to the list
	of keywords accepted in @property attribute contexts.
	* c-objc.h (enum objc_property_attribute_group): Add
	OBJC_PROPATTR_GROUP_CLASS.
	(enum objc_property_attribute_kind): Add
	OBJC_PROPERTY_ATTR_CLASS.

gcc/cp/ChangeLog:

	* parser.c (cp_parser_objc_at_property_declaration): Handle
	class keywords in @property attribute context.

gcc/objc/ChangeLog:

	* objc-act.c (objc_prop_attr_kind_for_rid): Handle class
	attribute.
	(objc_add_property_declaration): Likewise.
	* objc-act.h (PROPERTY_CLASS): Record class attribute state.

gcc/testsuite/ChangeLog:

	* obj-c++.dg/property/at-property-4.mm: Test handling class
	attributes.
	* objc.dg/property/at-property-4.m: Likewise.