Remove the second lifetime from DiagnosticArg.

Because it's always static.

I'm surprised the compiler allowed this unused lifetime without any
complaint.

compiler/rustc_errors/src/lib.rs

@@ -628,7 +628,7 @@ impl DiagCtxt {
     pub fn eagerly_translate<'a>(
         &self,
         message: DiagnosticMessage,
-        args: impl Iterator<Item = DiagnosticArg<'a, 'static>>,
+        args: impl Iterator<Item = DiagnosticArg<'a>>,
     ) -> SubdiagnosticMessage {
         SubdiagnosticMessage::Eager(Cow::from(self.eagerly_translate_to_string(message, args)))
     }
@@ -637,7 +637,7 @@ impl DiagCtxt {
     pub fn eagerly_translate_to_string<'a>(
         &self,
         message: DiagnosticMessage,
-        args: impl Iterator<Item = DiagnosticArg<'a, 'static>>,
+        args: impl Iterator<Item = DiagnosticArg<'a>>,
     ) -> String {
         let inner = self.inner.borrow();
         let args = crate::translation::to_fluent_args(args);