Match new parser behavior w.r.t. posonly args (#21891)

This matches behavior of the new parser to the old one. Not adding
tests, since some existing tests fail when switched to new parser, see
#21823

Author: ilevkivskyi

mypy/nativeparse.py

@@ -672,7 +672,7 @@ def read_func_def(state: State, data: ReadBuffer) -> FuncDef:
     name = read_str(data)
     arguments, has_ann = read_parameters(state, data)
 
-    if special_function_elide_names(name):
+    if state.options.pos_only_special_methods and special_function_elide_names(name):
         for arg in arguments:
             arg.pos_only = True