Migrate parts of rustc_expand to session diagnostics

This migrates everything but the `mbe` and `proc_macro` modules. It also
contains a few cleanups and drive-by/accidental diagnostic improvements
which can be seen in the diff for the UI tests.

compiler/rustc_expand/src/tests.rs

@@ -154,6 +154,7 @@ fn test_harness(file_text: &str, span_labels: Vec<SpanLabel>, expected_output: &
             false,
         );
         let handler = Handler::with_emitter(true, None, Box::new(emitter));
+        #[allow(rustc::untranslatable_diagnostic)]
         handler.span_err(msp, "foo");
 
         assert!(