Remove Session methods that duplicate DiagCtxt methods.

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

compiler/rustc_hir_analysis/src/impl_wf_check.rs

@@ -74,7 +74,7 @@ fn enforce_impl_params_are_constrained(tcx: TyCtxt<'_>, impl_def_id: LocalDefId)
     if impl_self_ty.references_error() {
         // Don't complain about unconstrained type params when self ty isn't known due to errors.
         // (#36836)
-        tcx.sess.span_delayed_bug(
+        tcx.dcx().span_delayed_bug(
             tcx.def_span(impl_def_id),
             format!(
                 "potentially unconstrained type parameters weren't evaluated: {impl_self_ty:?}",
@@ -171,7 +171,7 @@ fn enforce_impl_params_are_constrained(tcx: TyCtxt<'_>, impl_def_id: LocalDefId)
 
 fn report_unused_parameter(tcx: TyCtxt<'_>, span: Span, kind: &str, name: Symbol) {
     let mut err = struct_span_err!(
-        tcx.sess,
+        tcx.dcx(),
         span,
         E0207,
         "the {} parameter `{}` is not constrained by the \