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_builtin_macros/src/concat_bytes.rs

@@ -137,7 +137,7 @@ pub fn expand_concat_bytes(
     sp: rustc_span::Span,
     tts: TokenStream,
 ) -> Box<dyn base::MacResult + 'static> {
-    let Some(es) = base::get_exprs_from_tts(cx, sp, tts) else {
+    let Some(es) = base::get_exprs_from_tts(cx, tts) else {
         return DummyResult::any(sp);
     };
     let mut accumulator = Vec::new();