[OPENMP][NVPTX]Fixed processing of memory management directives.

Added special processing of the memory management directives/clauses for
NVPTX target. For private locals, omp_default_mem_alloc and
omp_thread_mem_alloc result in allocation in local memory.
omp_const_mem_alloc allocates const memory, omp_teams_mem_alloc
allocates shared memory, and omp_cgroup_mem_alloc and
omp_large_cap_mem_alloc allocate global memory.

llvm-svn: 357923