[flang][openacc] Fix post_alloc declare function ordering (#69980)

The declare actions were introduced to capture semantics dealing with
allocation of descriptor-based variable. However, the post_alloc action
has an ordering error. It needs to update descriptor first before the
mapping action of the data. The reason for this is that implicit attach
must occur during mapping action - but updating the descriptor
synchronizes it with the host copy (which would hold a host pointer).