Separate Analysis and Results.

`Results` contains and `Analysis` and an `EntryStates`. The unfortunate
thing about this is that the analysis needs to be mutable everywhere
(`&mut Analysis`) which forces the `Results` to be mutable everywhere,
even though `EntryStates` is immutable everywhere.

To fix this, this commit renames `Results` as `AnalysisAndResults`,
renames `EntryStates` as `Results`, and separates the analysis and
results as much as possible. (`AnalysisAndResults` doesn't get much use,
it's mostly there to facilitate method chaining of
`iterate_to_fixpoint`.)

`Results` is immutable everywhere, which:
- is a bit clearer on how the data is used,
- avoids an unnecessary clone of entry states in
  `locals_live_across_suspend_points`, and
- moves the results outside the `RefCell` in Formatter.

The commit also reformulates `ResultsHandle` as the generic `CowMut`,
which is simpler than `ResultsHandle` because it doesn't need the
`'tcx` lifetime and the trait bounds. It also which sits nicely
alongside the new use of `Cow` in `ResultsCursor`.

compiler/rustc_mir_dataflow/src/points.rs

@@ -98,7 +98,8 @@ rustc_index::newtype_index! {
 pub fn save_as_intervals<'tcx, N, A>(
     elements: &DenseLocationMap,
     body: &mir::Body<'tcx>,
-    mut results: Results<'tcx, A>,
+    mut analysis: A,
+    results: Results<A::Domain>,
 ) -> SparseIntervalMatrix<N, PointIndex>
 where
     N: Idx,
@@ -109,7 +110,8 @@ where
     visit_results(
         body,
         body.basic_blocks.reverse_postorder().iter().copied(),
-        &mut results,
+        &mut analysis,
+        &results,
         &mut visitor,
     );
     visitor.values