[OPENMP50]Add parsing/sema for acq_rel clause.

Added basic support (representation + parsing/sema/(de)serialization)
for acq_rel clause in flush/atomic directives.
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index 0921190..c25ce49 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -11666,6 +11666,9 @@
   case OMPC_seq_cst:
     C = new (Context) OMPSeqCstClause();
     break;
+  case OMPC_acq_rel:
+    C = new (Context) OMPAcqRelClause();
+    break;
   case OMPC_threads:
     C = new (Context) OMPThreadsClause();
     break;
@@ -11928,6 +11931,8 @@
 
 void OMPClauseReader::VisitOMPSeqCstClause(OMPSeqCstClause *) {}
 
+void OMPClauseReader::VisitOMPAcqRelClause(OMPAcqRelClause *) {}
+
 void OMPClauseReader::VisitOMPThreadsClause(OMPThreadsClause *) {}
 
 void OMPClauseReader::VisitOMPSIMDClause(OMPSIMDClause *) {}