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.

Here are the changes that I had to apply to the tests (they exposed
quite a number of interesting questions that I was happy to answer):

1) neg/t5845.scala now works, so I moved it to pos. That easily makes sense,
because the test was just a canary to track previous state of things.

2) neg/divergent_implicit.scala and neg/t5578.scala changed their messages
to less informative ones, because inapplicable implicit views now 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).

3) scaladoc/implicits-chaining.scala and scaladoc/implicits-base.scala.
Immediate culling of apriori inapplicable implicit views messes things up
for Scaladoc, because it's interested in potentially applicable views,
having infrastructure to track various constraints (type bounds, requirements
for implicit parameters, etc) that guide applicability of views and then
present it to the user.

4) We still cannot have Jason's workaround to type constructor inference
mentioned in comments to SI-3346, 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.