Simplify dataflow SwitchInt handling.

Current `SwitchInt` handling has complicated control flow.

- The dataflow engine calls `Analysis::apply_switch_int_edge_effects`,
  passing in an "applier" that impls `SwitchIntEdgeEffects`.
- `apply_switch_int_edge_effects` possibly calls `apply` on the applier,
  passing it a closure.
- The `apply` method calls the closure on each `SwitchInt` edge.
- The closure operates on the edge.

I.e. control flow goes from the engine, to the analysis, to the applier
(which came from the engine), to the closure (which came from the
analysis). It took me a while to work this out.

This commit changes to a simpler structure that maintains the important
characteristics.

- The dataflow engine calls `Analysis::get_switch_int_data`.
- `get_switch_int_data` returns an `Option<Self::SwitchIntData>` value.
- If that returned value was `Some`, the dataflow engine calls
  `Analysis::apply_switch_int_edge_effect` on each edge, passing the
  `Self::SwitchIntData` value.
- `Analysis::apply_switch_int_edge_effect` operates on the edge.

I.e. control flow goes from the engine, to the analysis, to the
engine, to the analysis.

Added:
- The `Analysis::SwitchIntData` assoc type and the
  `Analysis::get_switch_int_data` method. Both only need to be
  defined by analyses that look at `SwitchInt` terminators.
- The `MaybePlacesSwitchIntData` struct, which has three fields.

Changes:
- `Analysis::apply_switch_int_edge_effects` becomes
  `Analysis::apply_switch_int_edge_effect`, which is a little simpler
  because it's dealing with a single edge instead of all edges.

Removed:
- The `SwitchIntEdgeEffects` trait, and its two impls:
  `BackwardSwitchIntEdgeEffectsApplier` (which has six fields) and
  `ForwardSwitchIntEdgeEffectsApplier` structs (which has four fields).
- The closure.

The new structure is more concise and simpler.

compiler/rustc_borrowck/src/dataflow.rs

@@ -12,7 +12,7 @@ use rustc_mir_dataflow::impls::{
     EverInitializedPlaces, EverInitializedPlacesDomain, MaybeUninitializedPlaces,
     MaybeUninitializedPlacesDomain,
 };
-use rustc_mir_dataflow::{Analysis, GenKill, JoinSemiLattice, SwitchIntEdgeEffects};
+use rustc_mir_dataflow::{Analysis, GenKill, JoinSemiLattice};
 use tracing::debug;
 
 use crate::{BorrowSet, PlaceConflictBias, PlaceExt, RegionInferenceContext, places_conflict};
@@ -101,16 +101,6 @@ impl<'a, 'tcx> Analysis<'tcx> for Borrowck<'a, 'tcx> {
         // This is only reachable from `iterate_to_fixpoint`, which this analysis doesn't use.
         unreachable!();
     }
-
-    fn apply_switch_int_edge_effects(
-        &mut self,
-        _block: BasicBlock,
-        _discr: &mir::Operand<'tcx>,
-        _apply_edge_effects: &mut impl SwitchIntEdgeEffects<Self::Domain>,
-    ) {
-        // This is only reachable from `iterate_to_fixpoint`, which this analysis doesn't use.
-        unreachable!();
-    }
 }
 
 impl JoinSemiLattice for BorrowckDomain {