cmd/compile: better error message for when a type is in a constraint but not the type set
While at it, also remove the word "constraint" in the detail explanation
of an unsatisfied constraint.
Fixes #57500.
Change-Id: I55dae1694de2cfdb434aeba9d4a3530af7aca8f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/460455 Reviewed-by: Robert Findley <rfindley@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Robert Griesemer <gri@google.com>