[lldb] Fix data race in UnwindTable::Initialize (#197816)
UnwindTable::Initialize uses double-checked locking (DCL) against
m_scanned_all_unwind_sources, but the flag was a plain bool. The
fast-path read outside the mutex is unsynchronized with the write inside
the mutex, so concurrent first-time callers can observe partially
initialized state.
Make m_scanned_all_unwind_sources std::atomic<bool>. The default memory
ordering (seq_cst) is sufficient for the DCL pattern used here.
Found by ThreadSanitizer as part of #197792.
