C++: show private field accessor hints for const accesses (PR c++/84892)

C++: show private field accessor hints for const accesses (PR c++/84892)

gcc/cp/ChangeLog:
	PR c++/84892
	* search.c (field_accessor_p): Use class_of_this_parm rather than
	type_of_this_parm, to check that "this" is a "const T *", rather
	than a "T *const".

gcc/testsuite/ChangeLog:
	PR c++/84892
	* g++.dg/other/accessor-fixits-1.C
	(test_access_const_t1_color): New.
	(test_deref_const_t1_color): New.
	* g++.dg/other/accessor-fixits-5.C: New testcase.

From-SVN: r258716