[OPENMP50]Add initial support for OpenMP 5.0 iterator.

Added basic parsing/semantic analysis/(de)serialization support for
iterator expression introduced in OpenMP 5.0.
diff --git a/clang/lib/AST/NSAPI.cpp b/clang/lib/AST/NSAPI.cpp
index 2bfe977..3bb3605 100644
--- a/clang/lib/AST/NSAPI.cpp
+++ b/clang/lib/AST/NSAPI.cpp
@@ -484,6 +484,7 @@
   case BuiltinType::BuiltinFn:
   case BuiltinType::OMPArraySection:
   case BuiltinType::OMPArrayShaping:
+  case BuiltinType::OMPIterator:
     break;
   }