Remove ResultsVisitable.

Now that `Results` is the only impl of `ResultsVisitable`, the trait can
be removed. This simplifies things by removining unnecessary layers of
indirection and abstraction.

- `ResultsVisitor` is simpler.
  - Its type parameter changes from `R` (an analysis result) to the
    simpler `A` (an analysis).
  - It no longer needs the `Domain` associated type, because it can use
    `A::Domain`.
  - Occurrences of `R` become `Results<'tcx, A>`, because there is now
    only one kind of analysis results.

- `save_as_intervals` also changes type parameter from `R` to `A`.

- The `results.reconstruct_*` method calls are replaced with
  `results.analysis.apply_*` method calls, which are equivalent.

- `Direction::visit_results_in_block` is simpler, with a single generic
  param (`A`) instead of two (`D` and `R`/`F`, with a bound connecting
  them). Likewise for `visit_results`.

- The `ResultsVisitor` impls for `MirBorrowCtxt` and
  `StorageConflictVisitor` are now specific about the type of the
  analysis results they work with. They both used to have a type param
  `R` but they weren't genuinely generic. In both cases there was only a
  single results type that made sense to instantiate them with.

compiler/rustc_mir_dataflow/src/lib.rs

@@ -19,8 +19,8 @@ pub use self::drop_flag_effects::{
 };
 pub use self::framework::{
     Analysis, Backward, Direction, EntrySets, Forward, GenKill, JoinSemiLattice, MaybeReachable,
-    Results, ResultsCursor, ResultsVisitable, ResultsVisitor, SwitchIntEdgeEffects, fmt, graphviz,
-    lattice, visit_results,
+    Results, ResultsCursor, ResultsVisitor, SwitchIntEdgeEffects, fmt, graphviz, lattice,
+    visit_results,
 };
 use self::move_paths::MoveData;