Convert resume and retransopt to vasm

Summary: resume and retransopt were simply macro operations that do a
service request by preparing a number of argumets, saving
vmsp and vmfp, then returning to enterTCHelper with a service
request opcode.

Add a RegMask argument to the ret{} instruction so we can exactly
model the lifetime of the return-value registers and service-request
argument registers.

This only ports enough of the old emitServiceReqWork functions to
support what was needed for resume and retransopt. Adding support
for ephemeral service requests will take a bit more fleshing out.

Reviewed By: @swtaarrs

Differential Revision: D1622304
is_object to return false for incomplete classes

Summary: This incompliance must be fixed for Symfony's FlattenException to work (checked by Symfony/Component/Debug/Tests/Exception/FlattenExceptionTest.php).

Reviewed By: @jdelong

Differential Revision: D1574674
Make DateTime::modify work correctly

Summary: Right now, this code does not work in HHVM

```
<?php
$datetime = new DateTime();
$datetime->modify("2014-09-20 20:30");
echo $datetime->format('c');
```

It prints out the initial time upon construction as opposed to the modified time.

```
2014-10-16T17:29:03-07:00
```

PHP 5.6

```
2014-09-20T20:30:00-07:00
```

Fixes #3778

Reviewed By: @ptarjan

Differential Revision: D1622397
The HHIR CallsNative flag (N, NNT) is no longer needed.

Summary: Now that vasm-xls can see actual calls, we don't need the flag.

Reviewed By: @jdelong

Differential Revision: D1626565