[Modules] Fix another pointer keyed set that we iterate over while
writing a module to be a set-vector to preserve insertion order.

No test case, found by inspection.

llvm-svn: 233335
diff --git a/clang/include/clang/Serialization/ASTWriter.h b/clang/include/clang/Serialization/ASTWriter.h
index 41b1cab..bd01fc7 100644
--- a/clang/include/clang/Serialization/ASTWriter.h
+++ b/clang/include/clang/Serialization/ASTWriter.h
@@ -352,7 +352,7 @@
   /// if its primary namespace comes from the chain. If it does, we add the
   /// primary to this set, so that we can write out lexical content updates for
   /// it.
-  llvm::SmallPtrSet<const DeclContext *, 16> UpdatedDeclContexts;
+  llvm::SmallSetVector<const DeclContext *, 16> UpdatedDeclContexts;
 
   /// \brief Keeps track of visible decls that were added in DeclContexts
   /// coming from another AST file.