cmd/compile: fix package initialization ordering

This CL rewrites cmd/compile's package-level initialization ordering
algorithm to be compliant with the Go spec. See documentation in
initorder.go for details.

Incidentally, this CL also improves fidelity of initialization loop
diagnostics by including referenced functions in the emitted output
like go/types does.

Fixes #22326.

Change-Id: I7c9ac47ff563df4d4f700cf6195387a0f372cc7b
Reviewed-on: https://go-review.googlesource.com/c/go/+/170062
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -12,6 +12,7 @@ import (
 	"cmd/compile/internal/types"
 	"cmd/internal/obj"
 	"cmd/internal/src"
+	"sort"
 )
 
 // A Node is a single node in the syntax tree.
@@ -970,3 +971,30 @@ func (q *nodeQueue) popLeft() *Node {
 	q.head++
 	return n
 }
+
+// NodeSet is a set of Nodes.
+type NodeSet map[*Node]struct{}
+
+// Has reports whether s contains n.
+func (s NodeSet) Has(n *Node) bool {
+	_, isPresent := s[n]
+	return isPresent
+}
+
+// Add adds n to s.
+func (s *NodeSet) Add(n *Node) {
+	if *s == nil {
+		*s = make(map[*Node]struct{})
+	}
+	(*s)[n] = struct{}{}
+}
+
+// Sorted returns s sorted according to less.
+func (s NodeSet) Sorted(less func(*Node, *Node) bool) []*Node {
+	var res []*Node
+	for n := range s {
+		res = append(res, n)
+	}
+	sort.Slice(res, func(i, j int) bool { return less(res[i], res[j]) })
+	return res
+}