This is a replacement for the `absl_nullability_compatible` tag. The attribute
has the advantage that, unlike the tag, it can be applied to forward
declarations.

This does not yet change the implementation of the nullability annotations --
that will come in a followup patch.

As the nullability annotations themselves have not been changed, the
`absl_nullability_compatible` tag is currently still used to check whether the
annotations can be applied to a given type; see also the comments in the code.

PiperOrigin-RevId: 644238698
Change-Id: I5882606f82ce7a6dd98e83e6d920573437561b50