For a unification failure involving a constraint, rather than just
reporting (for instance)

        S does not match []E

now report the inferred type for the type parameter, use spec
terminology when referring to the constraint, and print the
constraint in full:

        S (type func()) does not satisfy ~[]E

There's more we can do, but this is better than what we had.

For #60542.

Change-Id: I033369fa0dfc475f0ec0da0582e8cbefb109f3cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/499639
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

In Checker.infer, report an error through an (incoming) *error_
so that the error can be reported as desired where infer is called.
Checker.infer is now a pure function.

Fixes golang#60543.

At call sites of Checker.infer, pass in an *error_ and use it to
report inference errors, together with additional information as
desired.

Fixes golang#60542.

In go/types, in error_.errorf, pass in a positioner rather than
a token.Pos. Also, introduce noposn, the positioner equivalent
for nopos. Adjust call sites as needed.

Change-Id: I462a7899a77a8bee2a21ba88299df237d74e0672
Reviewed-on: https://go-review.googlesource.com/c/go/+/558035
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>