[OPENMP50]Add if clause in distribute simd directive.

According to OpenMP 5.0, if clause can be used in for simd directive. If
condition in the if clause if false, the non-vectorized version of the
loop must be executed.
diff --git a/clang/lib/Basic/OpenMPKinds.cpp b/clang/lib/Basic/OpenMPKinds.cpp
index d95850f..c075deb 100644
--- a/clang/lib/Basic/OpenMPKinds.cpp
+++ b/clang/lib/Basic/OpenMPKinds.cpp
@@ -790,6 +790,8 @@
     }
     break;
   case OMPD_distribute_simd:
+    if (OpenMPVersion < 50 && CKind == OMPC_if)
+      return false;
     switch (CKind) {
 #define OPENMP_DISTRIBUTE_SIMD_CLAUSE(Name)                                    \
   case OMPC_##Name:                                                            \