[OpenMP] Use persistent memory for omp_large_cap_mem

This change enables volatile use of persistent memory for omp_large_cap_mem*
on supported systems. It depends on libmemkind's support for persistent memory,
and requirements/details can be found at the following url.

https://pmem.io/2020/01/20/memkind-dax-kmem.html

Differential Revision: https://reviews.llvm.org/D94353