Store a DefId instead of an AdtDef in AggregateKind::Adt

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.

compiler/rustc_mir_transform/src/shim.rs

@@ -777,7 +777,7 @@ pub fn build_adt_ctor(tcx: TyCtxt<'_>, ctor_id: DefId) -> Body<'_> {
         adt_def.variants[variant_index].fields.iter().enumerate().map(|(idx, field_def)| {
             (Operand::Move(Place::from(Local::new(idx + 1))), field_def.ty(tcx, substs))
         }),
-        AggregateKind::Adt(adt_def, variant_index, substs, None, None),
+        AggregateKind::Adt(adt_def.did, variant_index, substs, None, None),
         source_info,
         tcx,
     )