[C++ Patch] Improve check_special_function_return_type locations

[Jason Merrill]

On 6/5/19 11:06 AM, Paolo Carlini wrote:
> Hi,
> 
> here certainly we can do better than using input_location. In principle 
> we could also pass the location of the entity (constructor, destructor, 
> etc) itself or something else but I think it makes a lot of sense to 
> simply include locations[ds_type_spec] in the computation, seems 
> consistent with the existing case of spurious qualifiers (ICC does 
> something similar AFAICS). Tested x86_64-linux.

OK.

Jason