Change rustc_deprecated to use note

This keeps `reason` around for the time being. This is necessary to
avoid breakage during the bootstrap process. This change, as a whole,
brings `#[rustc_deprecated]` more in line with `#[deprecated]`.

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -438,6 +438,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                 .emit();
             }
         } else {
+            // FIXME(jhpratt) remove this after the two attributes are merged
             if attr.has_name(sym::deprecated) {
                 self.sess
                     .struct_span_err(attr.span, "`#[deprecated]` cannot be used in staged API")