[OpenMP][FIX] Allocate per launch memory for GPU team reductions (#70752)

We used to perform team reduction on global memory allocated in the
runtime and by clang. This was racy as multiple instances of a kernel,
or different kernels with team reductions, would use the same locations.
Since we now have the kernel launch environment, we can allocate dynamic
memory per-launch, allowing us to move all the state into a non-racy
place.

Fixes: https://github.com/llvm/llvm-project/issues/70249