SI-8868 Fix unpickling of local dummy symbols

These pop up as the owner of symbols in annotation arguments,
such as the ones introduced by the names/defaults desugaring.

The first two test cases here motivate the two patches to Unpicker.

The third requires both fixes, but exploits the problem directly,
without using `@deprecated` and named arguments.

See also 14fa7be / SI-8708 for a recently remedied kindred bug.

Review by `scala.annotation.`@lrytz