Remove NamedVarMap.

`NamedVarMap` is extremely similar to `ResolveBoundVars`. The former
contains two `UnordMap<ItemLocalId, T>` fields (obscured behind
`ItemLocalMap` typedefs). The latter contains two
`SortedMap<ItemLocalId, T>` fields. We construct a `NamedVarMap` and
then convert it into a `ResolveBoundVars` by sorting the `UnordMap`s,
which is unnecessary busywork.

This commit removes `NamedVarMap` and constructs a `ResolveBoundVars`
directly. `SortedMap` and `NamedVarMap` have slightly different
perf characteristics during construction (e.g. speed of insertion) but
this code isn't hot enough for that to matter.

A few details to note.
- A `FIXME` comment is removed.
- The detailed comments on the fields of `NamedVarMap` are copied to
  `ResolveBoundVars` (which has a single, incorrect comment).
- `BoundVarContext::map` is renamed.
- `ResolveBoundVars` gets a derived `Default` impl.

compiler/rustc_middle/src/middle/resolve_bound_vars.rs

@@ -45,15 +45,22 @@ pub enum ObjectLifetimeDefault {
     Param(DefId),
 }
 
-/// Maps the id of each lifetime reference to the lifetime decl
+/// Maps the id of each bound variable reference to the variable decl
 /// that it corresponds to.
-#[derive(HashStable, Debug)]
+#[derive(Debug, Default, HashStable)]
 pub struct ResolveBoundVars {
-    /// Maps from every use of a named (not anonymous) lifetime to a
-    /// `Region` describing how that region is bound
+    // Maps from every use of a named (not anonymous) bound var to a
+    // `ResolvedArg` describing how that variable is bound.
     pub defs: SortedMap<ItemLocalId, ResolvedArg>,
 
+    // Maps relevant hir items to the bound vars on them. These include:
+    // - function defs
+    // - function pointers
+    // - closures
+    // - trait refs
+    // - bound types (like `T` in `for<'a> T<'a>: Foo`)
     pub late_bound_vars: SortedMap<ItemLocalId, Vec<ty::BoundVariableKind>>,
 
+    // List captured variables for each opaque type.
     pub opaque_captured_lifetimes: LocalDefIdMap<Vec<(ResolvedArg, LocalDefId)>>,
 }