This commit removes the check that disallows the `#[non_exhaustive]`
attribute from being placed on enum variants and removes the associated
tests.

Further, this commit lowers the visibility of enum variant constructors
when the variant is marked as non-exhaustive.

This commit moves the `DefId` field of `Def::Ctor` to be the first
field.

This commit moves the definition of `CtorOf` from `rustc::hir` to
`rustc::hir::def` and adds imports wherever it is used.

This commit makes two changes - separating the `NodeId` that identifies
an enum variant from the `NodeId` that identifies the variant's
constructor; and no longer creating a `NodeId` for `Struct`-style enum
variants and structs.

Separation of the variant id and variant constructor id will allow the
rest of RFC 2008 to be implemented by lowering the visibility of the
variant's constructor without lowering the visbility of the variant
itself.

No longer creating a `NodeId` for `Struct`-style enum variants and
structs mostly simplifies logic as previously this `NodeId` wasn't used.
There were various cases where the `NodeId` wouldn't be used unless
there was an unit or tuple struct or enum variant but not all uses of
this `NodeId` had that condition, by removing this `NodeId`, this must
be explicitly dealt with. This change mostly applied cleanly, but there
were one or two cases in name resolution and one case in type check
where the existing logic required a id for `Struct`-style enum variants
and structs.

When failing to parse struct-like enum variants, the ADT gets recorded
as having no fields. Record that we have actually recovered during
parsing of this variant to avoid complaing about non-existing fields
when actually using it.

Report the diagnostic on macro expansions, and add a label indicating
why the comment is unused.

Co-Authored-By: NGabriel Smith <yodaldevoid@users.noreply.github.com>

Co-Authored-By: NGabriel Smith <yodaldevoid@users.noreply.github.com>