[Augmentations] Extension type augmentation must omit representation variable

[sgrekhov]

Augmentations specification was recently updated and now contains the following statement.

The representation field clause must be present on the declaration which introduces the extension type, and must be omitted from all augmentations of the extension type.

But analyzer doesn't support extension types augmentations without representation field and produces SYNTACTIC_ERROR.MISSING_PRIMARY_CONSTRUCTOR. Please, implement this functionality.

[pq]

@scheglov