go/types, types2: better error message for failing constraint type inference

go/types, types2: better error message for failing constraint type inference

We know the type argument against which constraint type inference fails:
print the type argument instead of the corresponding type parameter.

Fixes #57096.

Change-Id: Ia1da9c87fac6f8062e4d534b82e895fa4617fddc
Reviewed-on: https://go-review.googlesource.com/c/go/+/455278
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>