[ Runtime ] make `is` and `as` checks on enums not do string to int conversion

**Is your feature request related to a problem? Please describe.**

For backwards compat reasons, HHVM will do `is` and `as` checks on enums weakly.
This has some surprising results.

```HACK
enum Numberz: int {
ZERO = 0;
}

function main(): void {
$zero_as_a_string = '0';
if ($zero_as_a_string is Numberz) { // true
has_a_numberz_typehint($zero_as_a_string); // BOOM
}
}
```

This blows up at runtime, because `'0'` is not an interger.

**Describe the solution you'd like**

Make `is` and `as` checks fail if the runtime type doesn't match. `string <===> int`
We might need a ini setting to turn this to warn mode. (If it doesn't already exist.)

**Describe alternatives you've considered**

Back every enum with `arraykey` in your own code, so it doesn't fail at runtime.
However, this doesn't work for transparent (non-opaque) enums.

```HACK
enum Numbers: arraykey as int { }
```

I want to be able to use a `Numberz` as an integer, but this `arraykey as int` type constraint is invalid.

**Additional context**

[HHVM 3.28.0 release notes](https://hhvm.com/blog/2018/08/28/hhvm-3.28.0.html)
> For enums, the operators will validate that the value is in the given enum. CAUTION: the operators will perform integral key coercion to preserve compatibility with BuiltinEnum::isValid. We will fix this separately in the future.

Start by reproducing the enum example from the issue and compare the current behavior with the requested strict string-versus-integer check. The change is done when `is` and `as` reject mismatched runtime types without the documented integral key coercion; the possible warning-mode setting remains an unresolved design question.

Written by the indexing model from the issue text.