python: accept address and explicit locations in gdb.decode_line

The gdb.decode_line python function is documented to support the same location
expressions as the "break" command.  It currently expects a linespec location.

Instead of creating a linespec location directly, create the location via
string_to_event_location_basic.

gdb/python/python.c

@@ -740,7 +740,7 @@ gdbpy_decode_line (PyObject *self, PyObject *args)
 
   if (arg != NULL)
     {
-      location = new_linespec_location (&arg);
+      location = string_to_event_location_basic (&arg, python_language);
       make_cleanup_delete_event_location (location);
     }