Use a different DefId for the Def::Variant and Def::VariantCtor

[dlrobertson]

The enum variant (Dev::Variant) and the enum variants constructor (Def::VariantCtor) share the same DefId making it impossible for tcx.describe_def to resolve a DefId to a VariantCtor.

This issue originates from the discussion here.

Note: tcx.describe_def has not moved away from NodeId. It may be best for the fix to wait until it is.
CC: #59036

[davidtwco]

I think this can be closed. #59382 resolved this by separating the DefId used for a variant and its constructor. describe_def (now def_kind) can now return a Def::VariantCtor (now DefKind::Ctor) and the code from the original PR that raised this now uses DefKind::Ctor instead of DefKind::Variant as the review suggested.

[dlrobertson]

That is awesome! Thanks!