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_mir_transform/src/coverage/mod.rs

@@ -10,7 +10,6 @@ mod unexpand;
 
 use rustc_hir as hir;
 use rustc_hir::intravisit::{Visitor, walk_expr};
-use rustc_middle::hir::map::Map;
 use rustc_middle::hir::nested_filter;
 use rustc_middle::mir::coverage::{
     CoverageKind, DecisionInfo, FunctionCoverageInfo, Mapping, MappingKind,
@@ -348,7 +347,7 @@ fn extract_hole_spans_from_hir<'tcx>(
     hir_body: &hir::Body<'tcx>,
 ) -> Vec<Span> {
     struct HolesVisitor<'hir, F> {
-        hir: Map<'hir>,
+        tcx: TyCtxt<'hir>,
         visit_hole_span: F,
     }
 
@@ -360,8 +359,8 @@ fn extract_hole_spans_from_hir<'tcx>(
         ///   items contained within them.
         type NestedFilter = nested_filter::All;
 
-        fn nested_visit_map(&mut self) -> Self::Map {
-            self.hir
+        fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
+            self.tcx
         }
 
         fn visit_item(&mut self, item: &'hir hir::Item<'hir>) {
@@ -388,7 +387,7 @@ fn extract_hole_spans_from_hir<'tcx>(
 
     let mut hole_spans = vec![];
     let mut visitor = HolesVisitor {
-        hir: tcx.hir(),
+        tcx,
         visit_hole_span: |hole_span| {
             // Discard any holes that aren't directly visible within the body span.
             if body_span.contains(hole_span) && body_span.eq_ctxt(hole_span) {