Fix a possible crash in weakref_lru_cache.
I saw a crash during process destruction that I have been unable to reproduce, but after staring at the code the best mechanism I can think of is that clear() on a reentrant hash set isn't actually safe against reentrancy, so we have to be extremely careful that we do not trigger Python destructors from clear(). It looks like we were missing at least one case.
PiperOrigin-RevId: 886213343
