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/framework/graphviz.rs

@@ -544,20 +544,18 @@ impl<D> StateDiffCollector<D> {
     }
 }
 
-impl<'tcx, A> ResultsVisitor<'_, 'tcx, Results<'tcx, A>> for StateDiffCollector<A::Domain>
+impl<'tcx, A> ResultsVisitor<'_, 'tcx, A> for StateDiffCollector<A::Domain>
 where
     A: Analysis<'tcx>,
     A::Domain: DebugWithContext<A>,
 {
-    type Domain = A::Domain;
-
-    fn visit_block_start(&mut self, state: &Self::Domain) {
+    fn visit_block_start(&mut self, state: &A::Domain) {
         if A::Direction::IS_FORWARD {
             self.prev_state.clone_from(state);
         }
     }
 
-    fn visit_block_end(&mut self, state: &Self::Domain) {
+    fn visit_block_end(&mut self, state: &A::Domain) {
         if A::Direction::IS_BACKWARD {
             self.prev_state.clone_from(state);
         }
@@ -566,7 +564,7 @@ where
     fn visit_statement_before_primary_effect(
         &mut self,
         results: &mut Results<'tcx, A>,
-        state: &Self::Domain,
+        state: &A::Domain,
         _statement: &mir::Statement<'tcx>,
         _location: Location,
     ) {
@@ -579,7 +577,7 @@ where
     fn visit_statement_after_primary_effect(
         &mut self,
         results: &mut Results<'tcx, A>,
-        state: &Self::Domain,
+        state: &A::Domain,
         _statement: &mir::Statement<'tcx>,
         _location: Location,
     ) {
@@ -590,7 +588,7 @@ where
     fn visit_terminator_before_primary_effect(
         &mut self,
         results: &mut Results<'tcx, A>,
-        state: &Self::Domain,
+        state: &A::Domain,
         _terminator: &mir::Terminator<'tcx>,
         _location: Location,
     ) {
@@ -603,7 +601,7 @@ where
     fn visit_terminator_after_primary_effect(
         &mut self,
         results: &mut Results<'tcx, A>,
-        state: &Self::Domain,
+        state: &A::Domain,
         _terminator: &mir::Terminator<'tcx>,
         _location: Location,
     ) {