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_middle/src/middle/limits.rs

@@ -25,7 +25,7 @@ pub fn provide(providers: &mut ty::query::Providers) {
             tcx.hir().krate_attrs(),
             tcx.sess,
             sym::move_size_limit,
-            tcx.sess.opts.debugging_opts.move_size_limit.unwrap_or(0),
+            tcx.sess.opts.unstable_opts.move_size_limit.unwrap_or(0),
         ),
         type_length_limit: get_limit(
             tcx.hir().krate_attrs(),

compiler/rustc_middle/src/middle/stability.rs

@@ -443,7 +443,7 @@ impl<'tcx> TyCtxt<'tcx> {
                 // compiling a compiler crate), then let this missing feature
                 // annotation slide.
                 if feature == sym::rustc_private && issue == NonZeroU32::new(27812) {
-                    if self.sess.opts.debugging_opts.force_unstable_if_unmarked {
+                    if self.sess.opts.unstable_opts.force_unstable_if_unmarked {
                         return EvalResult::Allow;
                     }
                 }