It looks to me like the language rules treat these the same for this type, but evidently GCC feels differently.

This only matters under TSAN where SpinLock has a non-trivial destructor, and under C++20 where ABSL_CONST_INIT is implemented (as constinit) by gcc.

Fixes #1253

PiperOrigin-RevId: 469806751
Change-Id: Ic01b0142101f361bc19c95f9f9474e635669c58d