`repr_transparent_external_private_fields` lint is not totally clear about what exactly triggers the lint

[compiler-errors]

Seems fine to land this for now to fix the regression, but could you file a diagnostics issue about this? The error says "zero-sized fields [...] cannot contain external non-exhaustive types" but that's not really accurate, some zero-sized fields can in fact contain such non-exhaustive types.

Originally posted by @RalfJung in #115924 (comment)

The lint should be clearer about the exact situation where non-exhaustive 1-ZSTs can be in repr(transparent) structs. After #115924, errors fire more accurately, but error messages are still not totally right in their verbiage.