Adapt loplugin:derefnullptr to old Clang versions

...where prior to r183883 "Implement core issue 903: only integer literals with
value 0 and prvalues of type std::nullptr_t are null pointer constants from
C++11 onwards," Expr::isNullPointerConstant with NPC_NeverValueDependent could
go into an llvm_unreachable case.

Change-Id: I29cf093f18ece4cd83fd759e30f72c2a71f69554

compilerplugins/clang/derefnullptr.cxx

@@ -26,7 +26,7 @@ public:
 bool DerefNullPtr::VisitUnaryDeref(UnaryOperator const * op) {
     if (!ignoreLocation(op)
         && (op->getSubExpr()->IgnoreParenCasts()->isNullPointerConstant(
-                compiler.getASTContext(), Expr::NPC_NeverValueDependent/*TODO*/)
+                compiler.getASTContext(), Expr::NPC_ValueDependentIsNotNull/*TODO*/)
             != Expr::NPCK_NotNull))
     {
         report(