[OpenMP] Add API for pinned memory

This patch adds API support for the atk_pinned trait for omp_alloc.
It does not implement kmp_target_lock_mem and kmp_target_unlock_mem in libomptarget,
but prepares libomp for it. Patches to libomptarget to implement
lock/unlock coming after this one.

Reviewed by: jlpeyton, jdoerfert

Differential Revision: https://reviews.llvm.org/D138933