Remove Session methods that duplicate DiagCtxt methods.

Also add some `dcx` methods to types that wrap `TyCtxt`, for easier
access.

compiler/rustc_middle/src/util/bug.rs

@@ -31,8 +31,8 @@ fn opt_span_bug_fmt<S: Into<MultiSpan>>(
     tls::with_opt(move |tcx| {
         let msg = format!("{location}: {args}");
         match (tcx, span) {
-            (Some(tcx), Some(span)) => tcx.sess.dcx().span_bug(span, msg),
-            (Some(tcx), None) => tcx.sess.dcx().bug(msg),
+            (Some(tcx), Some(span)) => tcx.dcx().span_bug(span, msg),
+            (Some(tcx), None) => tcx.dcx().bug(msg),
             (None, _) => panic_any(msg),
         }
     })
@@ -42,7 +42,7 @@ fn opt_span_bug_fmt<S: Into<MultiSpan>>(
 /// `span_delayed_bug`, so what is the point of this? It exists to help us test `span_delayed_bug`'s
 /// interactions with the query system and incremental.
 pub fn trigger_span_delayed_bug(tcx: TyCtxt<'_>, key: rustc_hir::def_id::DefId) {
-    tcx.sess.span_delayed_bug(
+    tcx.dcx().span_delayed_bug(
         tcx.def_span(key),
         "delayed span bug triggered by #[rustc_error(span_delayed_bug_from_inside_query)]",
     );