The code was going through the raw `constraint_id` facet type, which
could be a named constraint. To get the interface being impl'd, use the
IdentifiedFacetType.
Import was adding an IdenfiedFacetTypeId for the facet type when
importing an ImplDecl, however it was using an attached self constant.
Then later lookups using `constant_values().GetConstantId(...)` from the
`self_id` would give an unattached constant and not find the
IdentifiedFacetTypeId. So have import do what we do when making an
ImplDecl locally, and use the unattached constant for the
RequireIdentifiedFacetType call.
We add a test of mangling an `impl as` for a named constraint, which
crashes before this change.
In some situations we need to mangle class declarations, which then are
not NameScopes (can't scope anything inside a declaration) - so make the
mangler able to cope with that situation by mangling the name of the
class directly rather than relying on generic NameScope mangling to
handle the class case.
Dana Jansens
David Blaikie