[lldb] Fix data race on ValueObject's unique-id counter (#197809)
g_value_obj_uid is a file-scope static that hands out unique IDs to
every ValueObject. It was a plain user_id_t, so concurrent
SBTarget::FindGlobalVariables / EvaluateExpression calls raced on the
increment.
Make it std::atomic<user_id_t>. Prefix operator++ on std::atomic is
already an atomic fetch_add that returns the new value, so the call
sites are unchanged.
Found by ThreadSanitizer as part of #197792.