Move some Map methods onto TyCtxt.

The end goal is to eliminate `Map` altogether.

I added a `hir_` prefix to all of them, that seemed simplest. The
exceptions are `module_items` which became `hir_module_free_items` because
there was already a `hir_module_items`, and `items` which became
`hir_free_items` for consistency with `hir_module_free_items`.

compiler/rustc_interface/src/proc_macro_decls.rs

@@ -7,7 +7,7 @@ use rustc_span::sym;
 fn proc_macro_decls_static(tcx: TyCtxt<'_>, (): ()) -> Option<LocalDefId> {
     let mut decls = None;
 
-    for id in tcx.hir().items() {
+    for id in tcx.hir_free_items() {
         let attrs = tcx.hir().attrs(id.hir_id());
         if attr::contains_name(attrs, sym::rustc_proc_macro_decls) {
             decls = Some(id.owner_id.def_id);