[flang][cuda][rt] Track asynchronous allocation stream for deallocation

[clementval]

When an asynchronous allocation is made, we call cudaMallocAsync with a stream. For deallocation, we need to call cudaFreeAsync with the same stream. in order to achieve that, we need to track the allocation and their respective stream.

This patch adds a simple sorted array of asynchronous allocations. A binary search is performed to retrieve the allocation when deallocation is needed.