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_mir_transform/src/const_debuginfo.rs

@@ -16,7 +16,7 @@ pub struct ConstDebugInfo;
 
 impl<'tcx> MirPass<'tcx> for ConstDebugInfo {
     fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
-        sess.opts.debugging_opts.unsound_mir_opts && sess.mir_opt_level() > 0
+        sess.opts.unstable_opts.unsound_mir_opts && sess.mir_opt_level() > 0
     }
 
     fn run_pass(&self, _tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {