[clang-tidy] add reproducer for PR39949 into test-suite

[JonasToth]

Summary

The underlying issue is fixed in https://reviews.llvm.org/D56444
and this test ensures the issue does not creep back into our
code-base.

[aaron.ballman]

LGTM, thank you for the extra test coverage!

test/clang-tidy/abseil-upgrade-duration-conversions.cpp

void factoryInMacros() {
   (void)absl::Nanoseconds(CONVERTIBLE_TMP);
   // CHECK-MESSAGES: [[@LINE-1]]:27: warning: implicit conversion to 'int64_t' is deprecated in this context; use an explicit cast instead
   // CHECK-FIXES: (void)absl::Nanoseconds(static_cast<int64_t>(CONVERTIBLE_TMP))
   ONLY_WARN_INSIDE_MACRO_FACTORY;
   // CHECK-MESSAGES: [[@LINE-1]]:3: warning: implicit conversion to 'int64_t' is deprecated in this context; use an explicit cast instead
   factoryTemplateAndMacro<ConvertibleTo<int>>();
   TemplateFactoryInMacro(ConvertibleTo<int>());
 }
+
+// This is a reduced test-case for PR39949 and manifested in this check.
+namespace std {
+template <typename _Tp>
+_Tp declval();
+
+template <typename _Functor, typename... _ArgTypes>
+struct __res {
+  template <typename... _Args>
+  static decltype(declval<_Functor>()(_Args()...)) _S_test(int);
+
+  template <typename...>
+  static void _S_test(...);
+
+  typedef decltype(_S_test<_ArgTypes...>(0)) type;
+};
+
+template <typename>
+struct function;
+
+template <typename... _ArgTypes>
+struct function<void(_ArgTypes...)> {
+  template <typename _Functor,
+            typename = typename __res<_Functor, _ArgTypes...>::type>
+  function(_Functor) {}
+};
+} // namespace std
+
+typedef std::function<void(void)> F;
+
+F foo() {
+  return F([] {});
+}