Only crate root def-ids don't have a parent, and in majority of cases the argument of `DefIdTree::parent` cannot be a crate root.
So we now panic by default in `parent` and introduce a new non-panicing function `opt_parent` for cases where the argument can be a crate root.
Same applies to `local_parent`/`opt_local_parent`.
This commit makes `AdtDef` use `Interned`. Much the commit is tedious
changes to introduce getter functions. The interesting changes are in
`compiler/rustc_middle/src/ty/adt.rs`.
The `AggregateKind` enum ends up in the final mir `Body`. Currently,
any changes to `AdtDef` (regardless of how significant they are)
will legitimately cause the overall result of `optimized_mir` to change,
invalidating any codegen re-use involving that mir.
This will get worse once we start hashing the `Span` inside `FieldDef`
(which is itself contained in `AdtDef`).
To try to reduce these kinds of invalidations, this commit changes
`AggregateKind::Adt` to store just the `DefId`, instead of the full
`AdtDef`. This allows the result of `optimized_mir` to be unchanged
if the `AdtDef` changes in a way that doesn't actually affect any
of the MIR we build.
