testsuite, Objective-c++ : Update diagnostic plugin test.

The @selector() and @protocol() operators produce var decls
these do not work with the example plugin.  Unfortunatetely,
unlike the ObjC front end, it is not so easy to construct a
substitute expression that works reliably.  Where it does not
work we xfail for now.

gcc/testsuite/ChangeLog:

	* obj-c++.dg/plugin/diagnostic-test-expressions-1.mm:
	Adjust testcase to include expressions for @selector and
	@protocol. XFAIL where this is still not sufficient to obtain
	a disgnostic range.