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_const_eval/src/transform/check_consts/check.rs

@@ -312,7 +312,7 @@ impl<'mir, 'tcx> Checker<'mir, 'tcx> {
             Status::Forbidden => None,
         };
 
-        if self.tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you {
+        if self.tcx.sess.opts.unstable_opts.unleash_the_miri_inside_of_you {
             self.tcx.sess.miri_unleashed_feature(span, gate);
             return;
         }