c++: Set the locus of the function result decl

c++: Set the locus of the function result decl

gcc/cp/ChangeLog:

	* decl.cc (grokdeclarator): Build RESULT_DECL.
	(start_preparsed_function): Copy location from template.
	* semantics.cc (apply_deduced_return_type): Handle
	arg != current_function_decl.
	* method.cc (implicitly_declare_fn): Use it.

gcc/ChangeLog:

	* function.cc (init_function_start): Use DECL_RESULT location
	for -Waggregate-return warning.

gcc/testsuite/ChangeLog:

	* g++.dg/diagnostic/return-type-loc1.C: New test.

Co-authored-by: Jason Merrill <jason@redhat.com>