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_const_eval/src/check_consts/check.rs

@@ -63,8 +63,7 @@ impl<'mir, 'tcx> Qualifs<'mir, 'tcx> {
             let ConstCx { tcx, body, .. } = *ccx;
 
             FlowSensitiveAnalysis::new(NeedsDrop, ccx)
-                .into_engine(tcx, body)
-                .iterate_to_fixpoint()
+                .iterate_to_fixpoint(tcx, body, None)
                 .into_results_cursor(body)
         });
 
@@ -93,8 +92,7 @@ impl<'mir, 'tcx> Qualifs<'mir, 'tcx> {
             let ConstCx { tcx, body, .. } = *ccx;
 
             FlowSensitiveAnalysis::new(NeedsNonConstDrop, ccx)
-                .into_engine(tcx, body)
-                .iterate_to_fixpoint()
+                .iterate_to_fixpoint(tcx, body, None)
                 .into_results_cursor(body)
         });
 
@@ -123,8 +121,7 @@ impl<'mir, 'tcx> Qualifs<'mir, 'tcx> {
             let ConstCx { tcx, body, .. } = *ccx;
 
             FlowSensitiveAnalysis::new(HasMutInterior, ccx)
-                .into_engine(tcx, body)
-                .iterate_to_fixpoint()
+                .iterate_to_fixpoint(tcx, body, None)
                 .into_results_cursor(body)
         });
 
@@ -239,8 +236,7 @@ impl<'mir, 'tcx> Checker<'mir, 'tcx> {
                 let always_live_locals = &always_storage_live_locals(&ccx.body);
                 let mut maybe_storage_live =
                     MaybeStorageLive::new(Cow::Borrowed(always_live_locals))
-                        .into_engine(ccx.tcx, &ccx.body)
-                        .iterate_to_fixpoint()
+                        .iterate_to_fixpoint(ccx.tcx, &ccx.body, None)
                         .into_results_cursor(&ccx.body);
 
                 // And then check all `Return` in the MIR, and if a local is "maybe live" at a