Ban all use of/references to the `__SystemLib` namespace outside of builtins

### HHVM Version

```
➜ nuxed git:(master) ✗ hhvm --version
HipHop VM 4.0.2 (rel)
Compiler: 1550605031_606999898
Repo schema: a46021ba5e51d676a396f711ef442e334fa9152a
➜ nuxed git:(master) ✗ hh_client --version
hh--4.0.2 Tue Feb 19 20:40:27 2019
➜ nuxed git:(master) ✗
```

### Operating System and Version

irrelevant

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

```
namespace __SystemLib;

<<__EntryPoint>>
async function enter_async_entry_point(): Awaitable {
}
```

### Actual result

```
➜ nuxed git:(master) ✗ hh_client foo.hack
No errors!
```

### Expected result

```Error : Cannot redeclare __SystemLib\enter_async_entry_point()```

or better :

```Error : Cannot use `__SystemLib` namespace.```

---

the issue here is that at run-time, you will get this :

```
➜ nuxed git:(master) ✗ hhvm foo.hack

Fatal error: Uncaught Error: Cannot redeclare __SystemLib\enter_async_entry_point() in :
Stack trace:
#0 {main}
```

Start with the provided foo.hack reproduction and compare hh_client's acceptance with hhvm's runtime fatal error. Trace how namespace declarations and the __EntryPoint function are handled in both paths; done means references to __SystemLib outside builtins are rejected before runtime.

Written by the indexing model from the issue text.