Add lint for invalid XHP class enum values

Add lint for invalid XHP class enum values

Summary:
If a user writes an XHP attribute with a literal value that isn't included in the enum, produce a lint warning. This warns on code of the form:

```
class :foo {
  attribute enum {'big', 'small'} size;
}

function bad_attr_value(): void {
  $x = <foo size="medium" />;
}
```

Reviewed By: hgoldstein

Differential Revision: D36728410

fbshipit-source-id: 0d54e32e8d3b2a757a0acc49fffbcc4aaa993be3