Make the test object callable. libstdc++'s bind checks that (libc++ currently does not). Thanks to Jonathan Wakely for the fix.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@359108 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/function.objects/bind/func.bind/func.bind.isbind/is_bind_expression.pass.cpp

@@ -25,7 +25,7 @@ test(const T&)
 #endif
 }
 
-struct C {};
+struct C {int operator()(...) const { return 0; }};
 
 int main(int, char**)
 {