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_hir_analysis/src/check/check.rs

@@ -435,8 +435,8 @@ fn best_definition_site_of_opaque<'tcx>(
     impl<'tcx> intravisit::Visitor<'tcx> for TaitConstraintLocator<'tcx> {
         type NestedFilter = nested_filter::All;
         type Result = ControlFlow<(Span, LocalDefId)>;
-        fn nested_visit_map(&mut self) -> Self::Map {
-            self.tcx.hir()
+        fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
+            self.tcx
         }
         fn visit_expr(&mut self, ex: &'tcx hir::Expr<'tcx>) -> Self::Result {
             if let hir::ExprKind::Closure(closure) = ex.kind {

compiler/rustc_hir_analysis/src/collect.rs

@@ -277,8 +277,8 @@ fn reject_placeholder_type_signatures_in_item<'tcx>(
 impl<'tcx> Visitor<'tcx> for CollectItemTypesVisitor<'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_hir_analysis/src/collect/dump.rs

@@ -63,8 +63,8 @@ pub(crate) fn def_parents(tcx: TyCtxt<'_>) {
             impl<'tcx> intravisit::Visitor<'tcx> for AnonConstFinder<'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_anon_const(&mut self, c: &'tcx rustc_hir::AnonConst) {

compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs

@@ -422,8 +422,8 @@ enum NonLifetimeBinderAllowed {
 impl<'a, 'tcx> Visitor<'tcx> for BoundVarContext<'a, '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_nested_body(&mut self, body: hir::BodyId) {

compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs

@@ -298,8 +298,8 @@ impl TaitConstraintLocator<'_> {
 impl<'tcx> intravisit::Visitor<'tcx> for TaitConstraintLocator<'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_expr(&mut self, ex: &'tcx Expr<'tcx>) {
         if let hir::ExprKind::Closure(closure) = ex.kind {
@@ -441,8 +441,8 @@ impl RpitConstraintChecker<'_> {
 impl<'tcx> intravisit::Visitor<'tcx> for RpitConstraintChecker<'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_expr(&mut self, ex: &'tcx Expr<'tcx>) {
         if let hir::ExprKind::Closure(closure) = ex.kind {