[dev.typeparams] cmd/compile/internal/types2: (TypeParam) SetBound -> SetConstraint

This matches the accessor named Constraint, and any documentation we have so far.
Use iface instead of Bound internally to types2; keep Bound because of two external
uses but mark it as deprecated. Adjust clients.

Change-Id: Id1a2c2f28259a16082e875eee0534d46cf157336
Reviewed-on: https://go-review.googlesource.com/c/go/+/338196


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