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_const_eval/src/transform/check_consts/qualifs.rs

@@ -270,7 +270,8 @@ where
         Rvalue::Aggregate(kind, operands) => {
             // Return early if we know that the struct or enum being constructed is always
             // qualified.
-            if let AggregateKind::Adt(def, _, substs, ..) = **kind {
+            if let AggregateKind::Adt(adt_did, _, substs, ..) = **kind {
+                let def = cx.tcx.adt_def(adt_did);
                 if Q::in_adt_inherently(cx, def, substs) {
                     return true;
                 }