core: Change XXXs into proper FIXMEs with issue numbers

src/libcore/stackwalk.rs

@@ -13,7 +13,8 @@
 #[legacy_modes]; // tjc: remove after snapshot
 
 // NB: transitionary, de-mode-ing.
-// XXX: Can't forbid this because frame_address needs a deprecated mode.
+// FIXME #4425: Can't forbid this because frame_address needs a deprecated
+// mode.
 #[allow(deprecated_mode)];
 #[forbid(deprecated_pattern)];