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.

compiler/rustc_mir_transform/src/lint.rs

@@ -17,13 +17,11 @@ pub(super) fn lint_body<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, when: String
     let always_live_locals = &always_storage_live_locals(body);
 
     let maybe_storage_live = MaybeStorageLive::new(Cow::Borrowed(always_live_locals))
-        .into_engine(tcx, body)
-        .iterate_to_fixpoint()
+        .iterate_to_fixpoint(tcx, body, None)
         .into_results_cursor(body);
 
     let maybe_storage_dead = MaybeStorageDead::new(Cow::Borrowed(always_live_locals))
-        .into_engine(tcx, body)
-        .iterate_to_fixpoint()
+        .iterate_to_fixpoint(tcx, body, None)
         .into_results_cursor(body);
 
     let mut lint = Lint {