Add #include, patch by Cédric Venet

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@44491 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/AST/Stmt.h

@@ -21,6 +21,7 @@
 #include "llvm/ADT/iterator"
 #include "llvm/Bitcode/SerializationFwd.h"
 #include <iosfwd>
+#include <string>
 
 using llvm::dyn_cast_or_null;