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, since it can't print anything
useful. Since we previously would derive Debug, lets just
keep it, to avoid needless breaking changes.

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
- forward-declaration-autoptr.rs: Manually implement clone.
  The inner type is unknown, so requiring clone/copy is not correct.

Author: jschwe