Arbitrary self types v2: adjust diagnostic.

The recently landed PR to adjust arbitrary self types was a bit
overenthusiastic, advising folks to use the new Receiver trait even
before it's been stabilized. Revert to the older wording of the lint in
such cases.

compiler/rustc_hir_analysis/src/errors.rs

@@ -1655,6 +1655,16 @@ pub(crate) struct NonConstRange {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(hir_analysis_invalid_receiver_ty_no_arbitrary_self_types, code = E0307)]
+#[note]
+#[help(hir_analysis_invalid_receiver_ty_help_no_arbitrary_self_types)]
+pub(crate) struct InvalidReceiverTyNoArbitrarySelfTypes<'tcx> {
+    #[primary_span]
+    pub span: Span,
+    pub receiver_ty: Ty<'tcx>,
+}
+
 #[derive(Diagnostic)]
 #[diag(hir_analysis_invalid_receiver_ty, code = E0307)]
 #[note]