[llvm-branch-commits] [tsan] Set `thr->is_inited` after SlotAttachAndLock (PR #86342)

[Vitaly Buka via llvm-branch-commits]

https://github.com/vitalybuka created https://github.com/llvm/llvm-project/pull/86342

Almost NFC.

This is symmetrical to `ThreadFinish`, which
resets the slot after `is_inited`.