Improve AdtDef interning.

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`.

compiler/rustc_middle/src/traits/chalk.rs

@@ -75,7 +75,7 @@ impl<'tcx> chalk_ir::interner::Interner for RustInterner<'tcx> {
     type InternedVariances = Vec<chalk_ir::Variance>;
     type InternedConstraints = Vec<chalk_ir::InEnvironment<chalk_ir::Constraint<Self>>>;
     type DefId = DefId;
-    type InternedAdtId = &'tcx AdtDef;
+    type InternedAdtId = AdtDef<'tcx>;
     type Identifier = ();
     type FnAbi = Abi;