b38ed1afa6
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.)