Overhaul Diagnostic args.

First, introduce a typedef `DiagnosticArgMap`.

Second, make the `args` field public, and remove the `args` getter and
`replace_args` setter. These were necessary previously because the getter
had a `#[allow(rustc::potential_query_instability)]` attribute, but that
was removed in #120931 when the args were changed from `FxHashMap` to
`FxIndexMap`. (All the other `Diagnostic` fields are public.)

compiler/rustc_errors/src/emitter.rs

@@ -519,7 +519,7 @@ impl Emitter for HumanEmitter {
     }
 
     fn emit_diagnostic(&mut self, mut diag: Diagnostic) {
-        let fluent_args = to_fluent_args(diag.args());
+        let fluent_args = to_fluent_args(diag.args.iter());
 
         let mut suggestions = diag.suggestions.unwrap_or(vec![]);
         self.primary_span_formatted(&mut diag.span, &mut suggestions, &fluent_args);