[PATCH, c++] tweak wording of `unknown type' in error messages

[Jason Merrill]

Alexandre Oliva wrote:
> Unknown type is not something that necessarily makes sense to end
> users, since it's just an internal representation detail of types yet
> to undergo overload resolution.  Some users seemed to be confused at
> it, so I figured tweaking the wording a bit could make the problem
> clearer.

> 	* error.c (dump_type) <UNKNOWN_TYPE>: Print nondeduced or unknown.

I think this change makes it even more confusing, since the type of the 
argument has nothing to do with template argument deduction.  I 
suggested <overloaded function>, which is in fact where we get 
UNKNOWN_TYPEs.

Jason