DataToTag instances in signature files don't work

Summary

Since !8912 (closed) landed, signature files are allowed to contain/request instances for the special DataToTag class. But these are not actually usable, and fail with a very unhelpful error message.

In principle there is no reason it shouldn't be possible to support DataToTag instances in signatures, but doing so doesn't appear completely trivial and there seems to be little or no demand for this feature.

See also the test case T15379-DataToTag.