[OPENMP50]Initial support for scan directive.

Addedi basic parsing/sema/serialization support for scan directive.
diff --git a/clang/lib/CodeGen/CGStmt.cpp b/clang/lib/CodeGen/CGStmt.cpp
index 49f1725..85c3bcc 100644
--- a/clang/lib/CodeGen/CGStmt.cpp
+++ b/clang/lib/CodeGen/CGStmt.cpp
@@ -250,6 +250,9 @@
   case Stmt::OMPDepobjDirectiveClass:
     EmitOMPDepobjDirective(cast<OMPDepobjDirective>(*S));
     break;
+  case Stmt::OMPScanDirectiveClass:
+    llvm_unreachable("Scan directive not supported yet.");
+    break;
   case Stmt::OMPOrderedDirectiveClass:
     EmitOMPOrderedDirective(cast<OMPOrderedDirective>(*S));
     break;