cmd/gc: Mark use of builtin functions as calls.

Fixes #4097.

R=rsc
CC=golang-dev, gri
https://golang.org/cl/6749059

src/cmd/gc/typecheck.c

@@ -245,6 +245,12 @@ callrecv(Node *n)
 	case OCALLINTER:
 	case OCALLFUNC:
 	case ORECV:
+	case OCAP:
+	case OLEN:
+	case OCOPY:
+	case ONEW:
+	case OAPPEND:
+	case ODELETE:
 		return 1;
 	}
 

test/fixedbugs/issue4097.go

@@ -0,0 +1,11 @@
+// errorcheck
+
+// Copyright 2012 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package foo
+
+var s [][10]int
+const m = len(s[len(s)-1]) // ERROR "must be constant" 
+