debug: Improve debug info of c++14 deduced return type [PR94459]

debug: Improve debug info of c++14 deduced return type [PR94459]

On the following testcase, in gdb ptype S<long>::m1 prints long as return
type, but all the other methods show void instead.
PR53756 added code to add_type_attribute if the return type is
auto/decltype(auto), but we actually should look through references,
pointers and qualifiers.
Haven't included there DW_TAG_atomic_type, because I think at least ATM
one can't use that in C++.  Not sure about DW_TAG_array_type or what else
could be deduced.

> http://eel.is/c++draft/dcl.spec.auto#3 says it has to appear as a
> decl-specifier.
>
> http://eel.is/c++draft/temp.deduct.type#8 lists the forms where a template
> argument can be deduced.
>
> Looks like you are missing arrays, pointers to members, and function return
> types.

2020-04-04  Hannes Domani  <ssbssa@yahoo.de>
	    Jakub Jelinek  <jakub@redhat.com>

	PR debug/94459
	* dwarf2out.c (gen_subprogram_die): Look through references, pointers,
	arrays, pointer-to-members, function types and qualifiers when
	checking if in-class DIE had an 'auto' or 'decltype(auto)' return type
	to emit type again on definition.

	* g++.dg/debug/pr94459.C: New test.

Co-Authored-By: Hannes Domani <ssbssa@yahoo.de>