cmd/compile/internal/syntax, types2: move cmpPos to pos.Cmp

Make position comparison generally available.

Change-Id: I94b6f658fa19a15b30574dbb2181879115c131a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/307215
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/syntax/pos.go

@@ -59,6 +59,45 @@ func (pos Pos) RelCol() uint {
 	return pos.Col()
 }
 
+// Cmp compares the positions p and q and returns a result r as follows:
+//
+//	r <  0: p is before q
+//	r == 0: p and q are the same position (but may not be identical)
+//	r >  0: p is after q
+//
+// If p and q are in different files, p is before q if the filename
+// of p sorts lexicographically before the filename of q.
+func (p Pos) Cmp(q Pos) int {
+	pname := p.RelFilename()
+	qname := q.RelFilename()
+	switch {
+	case pname < qname:
+		return -1
+	case pname > qname:
+		return +1
+	}
+
+	pline := p.Line()
+	qline := q.Line()
+	switch {
+	case pline < qline:
+		return -1
+	case pline > qline:
+		return +1
+	}
+
+	pcol := p.Col()
+	qcol := q.Col()
+	switch {
+	case pcol < qcol:
+		return -1
+	case pcol > qcol:
+		return +1
+	}
+
+	return 0
+}
+
 func (pos Pos) String() string {
 	rel := position_{pos.RelFilename(), pos.RelLine(), pos.RelCol()}
 	abs := position_{pos.Base().Pos().RelFilename(), pos.Line(), pos.Col()}