[lldb] Fix data race on Process::FindPlugin's unique-id counter (#197807)
g_process_unique_id is a function-local static used to hand out unique
IDs to each Process. It was a plain uint32_t, so concurrent
SBTarget::LoadCore / Target::CreateProcess calls raced on the increment.
Make it std::atomic<uint32_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.