swift

a90c7210 - use `compare_exchange_strong` to protect agianst spurious failures

use `compare_exchange_strong` to protect agianst spurious failures A `compare_exchange_weak` can spuriously return false, regardless of whether a concurrent access happened. This was causing a null-pointer dereference in TaskGroupImpl::poll in a narrow circumstance. The dereference failure only appears when using the `arm64` slice of the runtime library, since Clang will use `ldxr/stxr` for synchronization on such targets. The weak form does not retry on a spurious failure, but the strong version will. resolves rdar://84192672 (cherry picked from commit 69e80a1201636e124451de946dfa7fc255bf05ca)