[OpenMP] Synchronization hint constants added to headers

ident flags reserved for atomic hints.
This patch adds omp_sync_hint_t to omp.h and omp_sync_hint_kind to omp_lib.h.
For better maintainability the list of macros for ident flags was replaced with
a enum. The new KMP_IDENT_ATOMIC_HINT_MASK was added to the enum to
support possible future atomic hints.

Also fix omp_lib.h.var to be under 72 chars again after 5.0 OpenMP Memory commit

Patch by Olga Malysheva

Differential Revision: https://reviews.llvm.org/D51233

llvm-svn: 341693