Make DefIdForest cheaper to clone

Since `DefIdForest` contains 0 or 1 elements the large majority of the
time, by allocating only in the >1 case we avoid almost all allocations,
compared to `Arc<SmallVec<[DefId;1]>>`. This shaves off 0.2% on the
benchmark that stresses uninhabitedness checking.

compiler/rustc_middle/src/query/mod.rs

@@ -1314,7 +1314,7 @@ rustc_queries! {
         /// check whether the forest is empty.
         query type_uninhabited_from(
             key: ty::ParamEnvAnd<'tcx, Ty<'tcx>>
-        ) -> Arc<ty::inhabitedness::DefIdForest> {
+        ) -> ty::inhabitedness::DefIdForest {
             desc { "computing the inhabitedness of `{:?}`", key }
         }
     }