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_interface/src/queries.rs

@@ -357,7 +357,7 @@ impl Linker {
             return Ok(());
         }
 
-        if sess.opts.debugging_opts.no_link {
+        if sess.opts.unstable_opts.no_link {
             let encoded = CodegenResults::serialize_rlink(&codegen_results);
             let rlink_file = self.prepare_outputs.with_extension(config::RLINK_EXT);
             std::fs::write(&rlink_file, encoded).map_err(|err| {