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`.

clippy_lints/src/unconditional_recursion.rs

@@ -9,7 +9,6 @@ use rustc_hir::intravisit::{FnKind, Visitor, walk_body, walk_expr};
 use rustc_hir::{Body, Expr, ExprKind, FnDecl, HirId, Item, ItemKind, Node, QPath, TyKind};
 use rustc_hir_analysis::lower_ty;
 use rustc_lint::{LateContext, LateLintPass};
-use rustc_middle::hir::map::Map;
 use rustc_middle::hir::nested_filter;
 use rustc_middle::ty::{self, AssocKind, Ty, TyCtxt};
 use rustc_session::impl_lint_pass;
@@ -275,7 +274,6 @@ fn is_default_method_on_current_ty<'tcx>(tcx: TyCtxt<'tcx>, qpath: QPath<'tcx>,
 
 struct CheckCalls<'a, 'tcx> {
     cx: &'a LateContext<'tcx>,
-    map: Map<'tcx>,
     implemented_ty_id: DefId,
     method_span: Span,
 }
@@ -287,8 +285,8 @@ where
     type NestedFilter = nested_filter::OnlyBodies;
     type Result = ControlFlow<()>;
 
-    fn nested_visit_map(&mut self) -> Self::Map {
-        self.map
+    fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
+        self.cx.tcx
     }
 
     fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) -> ControlFlow<()> {
@@ -380,7 +378,6 @@ impl UnconditionalRecursion {
         {
             let mut c = CheckCalls {
                 cx,
-                map: cx.tcx.hir(),
                 implemented_ty_id,
                 method_span,
             };