Remove Session methods that duplicate DiagCtxt methods.

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

compiler/rustc_pattern_analysis/src/lints.rs

@@ -203,7 +203,7 @@ pub(crate) fn lint_nonexhaustive_missing_variants<'a, 'p, 'tcx>(
                 };
 
                 use rustc_errors::DecorateLint;
-                let mut err = rcx.tcx.sess.struct_span_warn(*arm.pat.data().unwrap(), "");
+                let mut err = rcx.tcx.dcx().struct_span_warn(*arm.pat.data().unwrap(), "");
                 err.set_primary_message(decorator.msg());
                 decorator.decorate_lint(&mut err);
                 err.emit();