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/lib.rs

@@ -10,6 +10,7 @@
 #![feature(rustc_attrs)]
 #![feature(try_blocks)]
 #![recursion_limit = "256"]
+#![deny(rustc::untranslatable_diagnostic)]
 
 #[macro_use]
 extern crate rustc_macros;
@@ -31,8 +32,13 @@ pub mod config;
 pub mod errors;
 pub mod expand;
 pub mod module;
+
+// FIXME(Nilstrieb) Translate proc_macro diagnostics
+#[allow(rustc::untranslatable_diagnostic)]
 pub mod proc_macro;
 
+// FIXME(Nilstrieb) Translate macro_rules diagnostics
+#[allow(rustc::untranslatable_diagnostic)]
 pub(crate) mod mbe;
 
 // HACK(Centril, #64197): These shouldn't really be here.