Remove Analysis::into_engine.

This is a standard pattern:
```
MyAnalysis.into_engine(tcx, body).iterate_to_fixpoint()
```
`into_engine` and `iterate_to_fixpoint` are always called in pairs, but
sometimes with a builder-style `pass_name` call between them. But a
builder-style interface is overkill here. This has been bugging me a for
a while.

This commit:
- Merges `Engine::new` and `Engine::iterate_to_fixpoint`. This removes
  the need for `Engine` to have fields, leaving it as a trivial type
  that the next commit will remove.
- Renames `Analysis::into_engine` as `Analysis::iterate_to_fixpoint`,
  gives it an extra argument for the optional pass name, and makes it
  call `Engine::iterate_to_fixpoint` instead of `Engine::new`.

This turns the pattern from above into this:
```
MyAnalysis.iterate_to_fixpoint(tcx, body, None)
```
which is shorter at every call site, and there's less plumbing required
to support it.

clippy_utils/src/mir/possible_borrower.rs

@@ -185,9 +185,7 @@ impl<'b, 'tcx> PossibleBorrowerMap<'b, 'tcx> {
             vis.into_map(cx)
         };
         let maybe_storage_live_result = MaybeStorageLive::new(Cow::Owned(BitSet::new_empty(mir.local_decls.len())))
-            .into_engine(cx.tcx, mir)
-            .pass_name("redundant_clone")
-            .iterate_to_fixpoint()
+            .iterate_to_fixpoint(cx.tcx, mir, Some("redundant_clone"))
             .into_results_cursor(mir);
         let mut vis = PossibleBorrowerVisitor::new(cx, mir, possible_origin);
         vis.visit_body(mir);