errors: only eagerly translate subdiagnostics

Subdiagnostics don't need to be lazily translated, they can always be
eagerly translated. Eager translation is slightly more complex as we need
to have a `DiagCtxt` available to perform the translation, which involves
slightly more threading of that context.

This slight increase in complexity should enable later simplifications -
like passing `DiagCtxt` into `AddToDiagnostic` and moving Fluent messages
into the diagnostic structs rather than having them in separate files
(working on that was what led to this change).

Signed-off-by: David Wood <david@davidtw.co>

compiler/rustc_errors/src/diagnostic_builder.rs

@@ -404,9 +404,6 @@ impl<'a, G: EmissionGuarantee> DiagnosticBuilder<'a, G> {
         name: impl Into<Cow<'static, str>>, arg: impl IntoDiagnosticArg,
     ));
     forward!((subdiagnostic, with_subdiagnostic)(
-        subdiagnostic: impl crate::AddToDiagnostic,
-    ));
-    forward!((eager_subdiagnostic, with_eager_subdiagnostic)(
         dcx: &DiagCtxt,
         subdiagnostic: impl crate::AddToDiagnostic,
     ));