[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/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index f7c58ed..27f44a7 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -6379,6 +6379,7 @@
   Record.push_back(C->varlist_size());
   Record.push_back(C->getNumLoops());
   Record.AddSourceLocation(C->getLParenLoc());
+  Record.AddStmt(C->getModifier());
   Record.push_back(C->getDependencyKind());
   Record.AddSourceLocation(C->getDependencyLoc());
   Record.AddSourceLocation(C->getColonLoc());