cmd/compile/internal/types2: review of pos.go and move into syntax package

This moves the two helper functions startPos and endPos into
the syntax package where they belong. Export the functions and
adjust dependent code.

Change-Id: I8170faeadd7cfa8f53009f81fcffd50ec0fc6a98
Reviewed-on: https://go-review.googlesource.com/c/go/+/305578


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>