[OPENMP50]Add if clause in for 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 b1e623b..eeeb8be 100644
--- a/clang/lib/Basic/OpenMPKinds.cpp
+++ b/clang/lib/Basic/OpenMPKinds.cpp
@@ -485,6 +485,8 @@
     }
     break;
   case OMPD_for_simd:
+    if (OpenMPVersion < 50 && CKind == OMPC_if)
+      return false;
     switch (CKind) {
 #define OPENMP_FOR_SIMD_CLAUSE(Name)                                           \
   case OMPC_##Name:                                                            \