error message formatting

[Joe Buck]

I wrote:
> > This one, on the other hand, may be trickier, because there are often
> > multiple typedefs referring to the same type (especially for maps and
> > sets: there is the map iterator as well as the rb_tree iterator).  It
> > might be tricky to decide which one to choose for display.

Per writes:
> You use the typedef name used in the declaration.

This suffices when an error is reported in a function: report the function
the way it was declared.  But in the case of a mismatched argument, there
are two declarations: the type of the actual argument and the type the
function was expecting.  There can be even more declarations involved when
there is an overloading ambiguity.

But maybe it's OK to just show everything the way it was declared.  It
would be worth trying out.