[ Feature Request ] Allow for a single throw statement as a body of an expression bodied lambda

### HHVM Version

4.27

### Operating System and Version

MacOS High Sierra

### Standalone code, or other way to reproduce the problem

```
<<__EntryPoint>>
function main(): void {
$_ = () ==> throw new \Exception();
}
```

### Actual result

```
$ hhvm test.hack

Fatal error: Uncaught Error: An expression is expected here. in /private/tmp/hhtest/test.hack:3
Stack trace:
#0 {main}
$ hh_client
Naming[2049] Unbound name: throw (a global constant)
--> test.hack
3 | $_ = () ==> throw new \Exception();
| ^^^^^

Typing[4106] Unbound global constant (Typing)
--> test.hack
3 | $_ = () ==> throw new \Exception();
| ^^^^^

Parsing[1002] An expression is expected here.
--> test.hack
3 | $_ = () ==> throw new \Exception();
|

3 errors found.
```

### Expected result

A lambda that throws when invoked.

Reproduce the issue from the standalone test.hack snippet, first running it through hhvm and hh_client to confirm the current parse and typing errors. Trace the handling of expression-bodied lambdas and throw syntax in the HHVM implementation. Done means the lambda is accepted and throws an exception when invoked, with hhvm and hh_client accepting the example.

Written by the indexing model from the issue text.