This diff adds logic to raise a warning whenever call_user_func_async() or
call_user_func_array_async() is invoked. The warning indicates that these
functions are deprecated, and suggests that fb_call_user_func_async() or
fb_call_user_func_array_async() should be used instead.
Write a small PHP program and verify the warning is raised.
Reviewed By: myang
Reviewers: myang, qigao, mwilliams, je, kma
CC: ps, mwilliams, jsobel, andrewparoski, myang
Differential Revision: 251155