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_build/src/build/expr/into.rs

@@ -377,7 +377,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     })
                 });
                 let adt = Box::new(AggregateKind::Adt(
-                    adt_def,
+                    adt_def.did,
                     variant_index,
                     substs,
                     user_ty,