Assignment of global variables deleted by their RHS in strict mode

[André Bargull]

> A fix for this would be in 8.1.1.4.5, insert between the current steps 4 and 5:
>
> 4.5  If S is true, then
>         a.   Let stillHasBinding be ObjRec.HasBinding(N).
>         b.   ReturnIfAbrupt(stillHasBinding).
>         c.   If stillHasBinding is false,  throw a ReferenceError exception.

I think the check should go into 8.1.1.2.5 SetMutableBinding. That way the similar issue for object
environment records gets also fixed:

```
var scope = {x: 1};

with (scope) {
  (function() {
    "use strict";
    x = (delete scope.x, 2);
  })();
}
```

Except for SpiderMonkey, all other engines tested (Edge, Nashorn, JSC, V8) already throw a
ReferenceError for the above snippet. But that's probably because engines don't implement the
'correct' Reference type semantics (https://bugs.ecmascript.org/show_bug.cgi?id=4379).


- André