[OpenMP] Use IsHostPtr where needed for targetDataBegin

As discussed in D105990, without this patch, `targetDataBegin`
determines whether to transfer data (as opposed to assuming it's in
shared memory) using the condition `!UseUSM || HasCloseModifier`.
However, this condition is broken if use of discrete memory was forced
by `omp_target_associate_ptr`.  This patch extends
`unified_shared_memory/associate_ptr.c` to reveal this case, and it
fixes it using `!IsHostPtr` in `DeviceTy::getTargetPointer` to replace
this condition.

Reviewed By: grokos

Differential Revision: https://reviews.llvm.org/D107927