[dev.typeparams] cmd/compile/internal/syntax: add Pos method

Allows syntax.Pos values to implement

interface {
	Pos() Pos
}

Preparation step for types2 package.

Change-Id: Ib0f4d7695a3d066983567d680fc3b9256a31c31d
Reviewed-on: https://go-review.googlesource.com/c/go/+/263622


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>