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/ty/closure.rs

@@ -164,7 +164,7 @@ impl<'tcx> CapturedPlace<'tcx> {
                         write!(
                             &mut symbol,
                             "__{}",
-                            def.variants[variant].fields[idx as usize].name.as_str(),
+                            def.variant(variant).fields[idx as usize].name.as_str(),
                         )
                         .unwrap();
                     }
@@ -330,7 +330,7 @@ pub fn place_to_string_for_capture<'tcx>(tcx: TyCtxt<'tcx>, place: &HirPlace<'tc
                     curr_string = format!(
                         "{}.{}",
                         curr_string,
-                        def.variants[variant].fields[idx as usize].name.as_str()
+                        def.variant(variant).fields[idx as usize].name.as_str()
                     );
                 }
                 ty::Tuple(_) => {