c++: adjust accessor fixits for explicit object parm

In a couple of places in the xobj patch I noticed that is_this_parameter
probably wanted to change to is_object_parameter; this implements that and
does the additional adjustments needed to make the accessor fixits handle
xobj parms.

gcc/cp/ChangeLog:

	* semantics.cc (is_object_parameter): New.
	* cp-tree.h (is_object_parameter): Declare.
	* call.cc (maybe_warn_class_memaccess): Use it.
	* search.cc (field_access_p): Use it.
	(class_of_object_parm): New.
	(field_accessor_p): Adjust for explicit object parms.

gcc/testsuite/ChangeLog:

	* g++.dg/torture/accessor-fixits-9-xobj.C: New test.