SI-3346 implicit parameters can now guide implicit view inference

This simple patch makes it possible for implicit views to benefit from
fundep-guided type inference, eliminating a nasty special case in
implicit inference.

We still cannot have Jason's workaround to type constructor inference,
because it's not really about implicit parameters of implicit views, but
about type inference flowing from the implicit parameter list to a
preceding parameter list in order to affect inference of an implicit view.
This is something that's still too ambitious for scalac.

I also had to modify some tests. neg/t5845.scala now works, so I moved
it to pos. That easily makes sense. The other two tests require some
explanation.

neg/divergent_implicit.scala and neg/t5578.scala changed their messages
to less informative ones, because inapplicable implicit views get
disqualified early and therefore don't display their error messages
to the user. This is an unfortunate but necessary byproduct of this
change, and one can argue that the behavior is now completely consistent
with implicit vals (that also don't explain why this or that implicit val
got disqualified, but rather display a generic implicit value not found
message).