Add an assertion to clone_momentary_breakpoint

This adds an assertion to clone_momentary_breakpoint.  This will
eventually be removed, but in the meantime is is useful for helping
convince oneself that momentary breakpoints will always use
momentary_breakpoint_ops.  This understanding will help when cleaning
up the code later.

gdb/breakpoint.c

@@ -7959,6 +7959,7 @@ clone_momentary_breakpoint (struct breakpoint *orig)
   if (orig == NULL)
     return NULL;
 
+  gdb_assert (orig->ops == &momentary_breakpoint_ops);
   return momentary_breakpoint_from_master (orig, orig->type, orig->ops, 0);
 }