bpo-36385: Add ``elif`` sentence on to avoid multiple ``if`` (GH-12478)

Currently, when arguments on Parser/asdl_c.py are parsed
``ìf`` sentence is used. This PR Propose to use ``elif``
to avoid multiple evaluting of the ifs.





https://bugs.python.org/issue36385

Parser/asdl_c.py

@@ -1313,9 +1313,9 @@ if __name__ == "__main__":
     for o, v in opts:
         if o == '-h':
             H_FILE = v
-        if o == '-c':
+        elif o == '-c':
             C_FILE = v
-        if o == '-d':
+        elif o == '-d':
             dump_module = True
     if H_FILE and C_FILE:
         print('Must specify exactly one output file')