[libomptarget] Keep the Shadow Pointer Map up-to-date

D105812 introduced a regression where if a PTR_AND_OBJ entry was mapped on the device, then the OBJ was deallocated and then reallocated at a different address, the Shadow Pointer Map would still contain an entry for the PTR but pointing to the old address. This caused test `env/base_ptr_ref_count.c` to fail.

Differential Revision: https://reviews.llvm.org/D105947