Add fix-it hint for missing return statement in assignment operators (PR c++/85523)

Add fix-it hint for missing return statement in assignment operators (PR c++/85523)

gcc/cp/ChangeLog:

	PR c++/85523
	* decl.c: Include "gcc-rich-location.h".
	(add_return_star_this_fixit): New function.
	(finish_function): When warning about missing return statements in
	functions returning non-void, add a "return *this;" fix-it hint for
	assignment operators.

gcc/testsuite/ChangeLog:

	PR c++/85523
	* g++.dg/pr85523.C: New test.

Co-Authored-By: Jonathan Wakely <jwakely@redhat.com>
From-SVN: r263298