gh-120321: Make gen.gi_frame.clear() thread-safe

[colesbury]

This splits up the clearing of a generator frame from the tp_finalize implementation, which makes it easier to do the frame transition atomically.

Note that since gh-111792, it's only legal to call generator.gi_frame.clear() on a generator in the FRAME_CREATED state or finished state, not any of the suspended states.

• Issue: SIGSEGV with generators in free-threaded build #120321

[colesbury]

I've seen this reported in TSan, although I'm not sure where the frame.clear() call is happening in Python:

test_imap_handle_iterable_exception (test.test_multiprocessing_forkserver.test_threads.WithThreadsTestPool.test_imap_handle_iterable_exception) ... ==================
WARNING: ThreadSanitizer: data race (pid=952942)
  Atomic write of size 1 at 0x7fffb56c24a3 by thread T79:
    #0 _Py_atomic_compare_exchange_int8 /home/sgross/cpython/./Include/cpython/pyatomic_gcc.h:75:10 (python+0x2ba69c) (BuildId: 59de1379ec77c44e6bf58e50950f5e82982a1cca)
    #1 gen_send_ex /home/sgross/cpython/Objects/genobject.c:327:15 (python+0x2ba69c)
    #2 gen_iternext /home/sgross/cpython/Objects/genobject.c:721:9 (python+0x2b6ec2) (BuildId: 59de1379ec77c44e6bf58e50950f5e82982a1cca)
    #3 _PyForIter_VirtualIteratorNext /home/sgross/cpython/Python/ceval.c:4115:24 (python+0x527188) (BuildId: 59de1379ec77c44e6bf58e50950f5e82982a1cca)
    #4 _PyEval_EvalFrameDefault /home/sgross/cpython/Python/generated_cases.c.h:5375:36 (python+0x50aa13) (BuildId: 59de1379ec77c44e6bf58e50950f5e82982a1cca)
    #5 _PyEval_EvalFrame /home/sgross/cpython/./Include/internal/pycore_ceval.h:119:16 (python+0x4ec83d) (BuildId: 59de1379ec77c44e6bf58e50950f5e82982a1cca)
...

  Previous read of size 1 at 0x7fffb56c24a3 by main thread:
    #0 frame_clear_impl /home/sgross/cpython/Objects/frameobject.c:2019:18 (python+0x2d06f2) (BuildId: 59de1379ec77c44e6bf58e50950f5e82982a1cca)
    #1 frame_clear /home/sgross/cpython/Objects/clinic/frameobject.c.h:407:20 (python+0x2d06f2)
    #2 _PyEval_EvalFrameDefault /home/sgross/cpython/Python/generated_cases.c.h:3556:35 (python+0x50b406) (BuildId: 59de1379ec77c44e6bf58e50950f5e82982a1cca)
    #3 _PyEval_EvalFrame /home/sgross/cpython/./Include/internal/pycore_ceval.h:119:16 (python+0x4ec83d) (BuildId: 59de1379ec77c44e6bf58e50950f5e82982a1cca)
...