[dev.inline] cmd/compile/internal/syntax: add predicates to syntax.Pos

This moves syntax.Pos closer to cmd/internal/src.Pos so that
we can more easily replace src.Pos with syntax.Pos going forward.

Change-Id: I9f93a65fecb4c22591edca4b9d6cda39cf0e872e
Reviewed-on: https://go-review.googlesource.com/34270


Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>