interp: fix support of type assert expressions in the global scope

Add the support of type assert expressions in type parsing
which allows to process type assertions in the global scope.

Fixes #1543.

_test/assert4.go

@@ -0,0 +1,11 @@
+package main
+
+var cc interface{} = 2
+var dd = cc.(int)
+
+func main() {
+	println(dd)
+}
+
+// Output:
+// 2

interp/type.go

@@ -1093,6 +1093,9 @@ func nodeType2(interp *Interpreter, sc *scope, n *node, seen []*node) (t *itype,
 			sc.sym[sname].typ = t
 		}
 
+	case typeAssertExpr:
+		t, err = nodeType2(interp, sc, n.child[1], seen)
+
 	default:
 		err = n.cfgErrorf("type definition not implemented: %s", n.kind)
 	}