replace new ThinVec with clear()

Co-authored-by: León Orell Valerian Liehr <[email protected]>

Author: PonasKovas

compiler/rustc_builtin_macros/src/deriving/generic/mod.rs

@@ -683,7 +683,7 @@ impl<'a> TraitDef<'a> {
             .map(|mut param| {
                 // Remove all attributes, because there might be helper attributes
                 // from other macros that will not be valid in the expanded implementation.
-                param.attrs = ThinVec::new();
+                param.attrs.clear();
                 param
             })
             .collect();