go/types: fix conversions of constants to type parameter

This is a port of both CL 360396 and CL 360796 to go/types. The latter
is added to avoid introducing an intermediate bug.

An error message was adjusted in issue49296.go2, with a TODO to switch
to the types2 error.

Change-Id: Iede294b69b4e097e53876498f039ee18667568c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/360934


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>