A few small hhbbc tweaks

Summary: - reduce BoxR to BoxRNop
   - constprop CGetS
   - reduce InstanceOf to InstanceOfD in one more case
   - allow dce to remove AGetC

Reviewed By: @elgenie

Differential Revision: D1415312
Always mark Closure classes as NoOverride

Summary: Closures cannot be extended, so we can mark them as NoOverride even in
non-repo modes.

Reviewed By: @jdelong

Differential Revision: D1442144
Remove XMLWriter test temporarily

Summary: Seems relatively harmless, likely cauesd by HNI conversion. Will investigate reverting this immediately.

```
[fredemmottdevbig076 ~/fbcode/hphp/test] php5 quick/builtin_extension_XMLWriter.php | grep -i startattributens
  string(16) "startAttributeNs"
  string(16) "startAttributeNs"
[fredemmottdevbig076 ~/fbcode/hphp/test] hhvm quick/builtin_extension_XMLWriter.php | grep -i startattributens
  string(16) "startAttributens"
  string(16) "startAttributens"
```

Reviewed By: bsimmers, ptarjan

Differential Revision: D1444383
clean up libxml/dom/simplexml

Summary: - Move 2 php_libxml_* functions to their appropriate place to
  reflect Zend (libxml extension).
- Call these functions from SimpleXML to properly clean up after
  ourselves, which earlier was incorrect assumed to be entirely
  handled by xmlFreeDoc in sweeps.
- Don't NULL out node->doc, this should only be done at refcount 0 to
  prevent double frees. XML extensions take care of this themselves.

There's still a bunch of work needed to be done: SimpleXML and
DOMDocument share a lot more functionality that should eventually be
moved to libxml as well.

That will be a gradual process however, and this change plugs a leak in
the meantime.

Closes https://github.com/facebook/hhvm/pull/3223

Reviewed By: @ptarjan

Differential Revision: D1443782