cmd/compile: mark memclrHasPointers calls as write barriers

There are two places where the compiler generates memclrHasPointers
calls. These are effectively write barriers, but the compiler doesn't
currently record them as such in the function. As a result code like

  for i := range a {
    a[i] = nil
  }

inserts a write barrier for the assignment to a[i], but the compiler
doesn't report this. Hence, it's not reported in the -d=wb output, and
it's not checked against //go:nowritebarrier annotations.

Change-Id: I40299ebc9824f05cf516cba494d4c086b80ffb53
Reviewed-on: https://go-review.googlesource.com/c/152722
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -588,6 +588,7 @@ func arrayClear(n, v1, v2, a *Node) bool {
 	var fn *Node
 	if types.Haspointers(a.Type.Elem()) {
 		// memclrHasPointers(hp, hn)
+		Curfn.Func.setWBPos(stmt.Pos)
 		fn = mkcall("memclrHasPointers", nil, nil, hp, hn)
 	} else {
 		// memclrNoHeapPointers(hp, hn)