[Libomptarget] Implement OpenMP 5.2 semantics for device pointers

In OpenMP 5.2, §5.8.6, page 160 line 32-33, when a device pointer
allocated by omp_target_alloc has implicitly been included on a target
construct as a zero-length array, the pointer initialisation should not
find a matching mapped list item, and so should retain its value as a
firstprivate variable. Previously, we would return a null pointer if the
list item was not found. This patch updates the map handling to the
OpenMP 5.2 semantics.

Reviewed By: jdoerfert, ye-luo

Differential Revision: https://reviews.llvm.org/D133447