[OpenMP][libomptarget] Delay restore of shadow pointers in structs to after...

[OpenMP][libomptarget] Delay restore of shadow pointers in structs to after H2D memory copies are completed

When using asynchronous plugin calls, shadow pointer restore could happen before the D2H copy for the entire struct has completed, effectively leaving a device pointer in a host struct.
This patch fixes the problem by delaying restore's to after a synchronization happens (target regions) and by calling early synchronization (target update).

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D119968