[dev.typeparams] go/types: (TypeParam) SetBound -> SetConstraint

[dev.typeparams] go/types: (TypeParam) SetBound -> SetConstraint

This is a straightforward port of CL 338196 to go/types, minus the
deprecated TypeParam.Bound() method (since it is not needed), plus an
adjustment for methodset.go.

Change-Id: Ie372bfeec245094102a2c3257a43499d75981447
Reviewed-on: https://go-review.googlesource.com/c/go/+/339675
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>