fb_enable_code_coverage needs an ActRec

It tries to unwind the top, builtin frame, making
various assumptions that aren't true if it was called by
FCallBuiltin. In dbg builds, it asserts.

hphp/system/idl/fb.idl.json

@@ -584,7 +584,8 @@
             "desc": "Enables code coverage. The coverage information is cleared.",
             "flags": [
                 "HasDocComment",
-                "HipHopSpecific"
+                "HipHopSpecific",
+                "NeedsActRec"
             ],
             "return": {
                 "type": null

hphp/test/slow/code_coverage/check_actrec.php

@@ -0,0 +1,9 @@
+<?php
+
+function test($t) {
+  fb_enable_code_coverage();
+  var_dump($t);
+  fb_disable_code_coverage();
+}
+
+test("hello");

hphp/test/slow/code_coverage/check_actrec.php.expect

@@ -0,0 +1 @@
+string(5) "hello"