Prevent default derives for Forward declared types.

We do not know the layout for forward declared types.
Deriving Copy, Clone, Hash, PartialEq etc. is simply wrong.
Deriving Debug is debatable. The default derive might give a wrong
impression, since it prints out the inner unused field.
The opt-in manual implementation would print opaque, which seems
friendlier.

Changed tests:
- func-return-must-use: Don't use forward declared structs in the
  test, since that would result in wrong bindings!
- issue-1238-fwd-no-copy test no longer requires an annotation,
  since Forward declared types should always be no copy

Author: jschwe