Validate OpForwardPointer (#2156)

* Validate OpForwardPointer

The validator does not have a a check that OpForwardPointer is giving
a forward reference to a pointer type.  We add that check.

https://crbug.com/910852

* Remove more specialized check.

There was a check that the forward pointer is actually a poiner type,
but it was only done if it was used in a struct.  This was too specific.
Remove it in favour of the more general check that was added.

* Format

* Check the storage type in OpTypeForwardPointer

* Fix typo is test case epxected results.