Rename debugging_opts to unstable_opts

This is no longer used only for debugging options (e.g. `-Zoutput-width`, `-Zallow-features`).
Rename it to be more clear.

compiler/rustc_trait_selection/src/traits/engine.rs

@@ -9,7 +9,7 @@ pub trait TraitEngineExt<'tcx> {
 
 impl<'tcx> TraitEngineExt<'tcx> for dyn TraitEngine<'tcx> {
     fn new(tcx: TyCtxt<'tcx>) -> Box<Self> {
-        if tcx.sess.opts.debugging_opts.chalk {
+        if tcx.sess.opts.unstable_opts.chalk {
             Box::new(ChalkFulfillmentContext::new())
         } else {
             Box::new(FulfillmentContext::new())