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_ty_utils/src/ty.rs

@@ -126,7 +126,7 @@ fn param_env(tcx: TyCtxt<'_>, def_id: DefId) -> ty::ParamEnv<'_> {
     // are any errors at that point, so outside of type inference you can be
     // sure that this will succeed without errors anyway.
 
-    if tcx.sess.opts.debugging_opts.chalk {
+    if tcx.sess.opts.unstable_opts.chalk {
         let environment = well_formed_types_in_env(tcx, def_id);
         predicates.extend(environment);
     }