Overhaul the intravisit::Map trait.

First of all, note that `Map` has three different relevant meanings.
- The `intravisit::Map` trait.
- The `map::Map` struct.
- The `NestedFilter::Map` associated type.

The `intravisit::Map` trait is impl'd twice.
- For `!`, where the methods are all unreachable.
- For `map::Map`, which gets HIR stuff from the `TyCtxt`.

As part of getting rid of `map::Map`, this commit changes `impl
intravisit::Map for map::Map` to `impl intravisit::Map for TyCtxt`. It's
fairly straightforward except various things are renamed, because the
existing names would no longer have made sense.

- `trait intravisit::Map` becomes `trait intravisit::HirTyCtxt`, so named
  because it gets some HIR stuff from a `TyCtxt`.
- `NestedFilter::Map` assoc type becomes `NestedFilter::MaybeTyCtxt`,
  because it's always `!` or `TyCtxt`.
- `Visitor::nested_visit_map` becomes `Visitor::maybe_tcx`.

I deliberately made the new trait and associated type names different to
avoid the old `type Map: Map` situation, which I found confusing. We now
have `type MaybeTyCtxt: HirTyCtxt`.

compiler/rustc_incremental/src/assert_dep_graph.rs

@@ -174,8 +174,8 @@ impl<'tcx> IfThisChanged<'tcx> {
 impl<'tcx> Visitor<'tcx> for IfThisChanged<'tcx> {
     type NestedFilter = nested_filter::OnlyBodies;
 
-    fn nested_visit_map(&mut self) -> Self::Map {
-        self.tcx.hir()
+    fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
+        self.tcx
     }
 
     fn visit_item(&mut self, item: &'tcx hir::Item<'tcx>) {

compiler/rustc_incremental/src/persist/dirty_clean.rs

@@ -455,8 +455,8 @@ impl<'tcx> FindAllAttrs<'tcx> {
 impl<'tcx> intravisit::Visitor<'tcx> for FindAllAttrs<'tcx> {
     type NestedFilter = nested_filter::All;
 
-    fn nested_visit_map(&mut self) -> Self::Map {
-        self.tcx.hir()
+    fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
+        self.tcx
     }
 
     fn visit_attribute(&mut self, attr: &'tcx Attribute) {