cmd/cgo: add missing ast.SliceExpr.Max case to AST traversal.

+ static test

NB: there's a preexisting (dynamic) failure of test issue7978.go.

LGTM=iant
R=rsc, iant
CC=golang-codereviews
https://golang.org/cl/144650045

misc/cgo/test/basic.go

@@ -157,3 +157,8 @@ func testUnsignedInt(t *testing.T) {
 		t.Errorf("Incorrect unsigned int - got %x, want %x", a, b)
 	}
 }
+
+// Static (build-time) test that syntax traversal visits all operands of s[i:j:k].
+func sliceOperands(array [2000]int) {
+	_ = array[C.KILO:C.KILO:C.KILO] // no type error
+}

src/cmd/cgo/ast.go

@@ -308,6 +308,9 @@ func (f *File) walk(x interface{}, context string, visit func(*File, interface{}
 		if n.High != nil {
 			f.walk(&n.High, "expr", visit)
 		}
+		if n.Max != nil {
+			f.walk(&n.Max, "expr", visit)
+		}
 	case *ast.TypeAssertExpr:
 		f.walk(&n.X, "expr", visit)
 		f.walk(&n.Type, "type", visit)