[OPENMP50]Add support for 'release' clause.

Added full support for 'release' clause in flush|atomic directives.
diff --git a/clang/lib/Parse/ParseOpenMP.cpp b/clang/lib/Parse/ParseOpenMP.cpp
index 0214a6a..1a7916e 100644
--- a/clang/lib/Parse/ParseOpenMP.cpp
+++ b/clang/lib/Parse/ParseOpenMP.cpp
@@ -2090,7 +2090,7 @@
 ///       nogroup-clause | num_tasks-clause | hint-clause | to-clause |
 ///       from-clause | is_device_ptr-clause | task_reduction-clause |
 ///       in_reduction-clause | allocator-clause | allocate-clause |
-///       acq_rel-clause | acquire-clause
+///       acq_rel-clause | acquire-clause | release-clause
 ///
 OMPClause *Parser::ParseOpenMPClause(OpenMPDirectiveKind DKind,
                                      OpenMPClauseKind CKind, bool FirstClause) {
@@ -2201,6 +2201,7 @@
   case OMPC_seq_cst:
   case OMPC_acq_rel:
   case OMPC_acquire:
+  case OMPC_release:
   case OMPC_threads:
   case OMPC_simd:
   case OMPC_nogroup: