[ADT] Replace `void_t` equivalent with `std::void_t`

[jloser]

Summary

Use std::void_t instead of defining our own equivalent in STLExtras.h now
that C++17 is available for use.

[jloser]

Rebase to make sure CI failure is unrelated

llvm/include/llvm/ADT/STLExtras.h

 };
 
 template <typename T> struct make_const_ref {
   using type = typename std::add_lvalue_reference<
       typename std::add_const<T>::type>::type;
 };
 
 namespace detail {
-template <typename...> using void_t = void;
 template <class, template <class...> class Op, class... Args> struct detector {
   using value_t = std::false_type;
 };
 template <template <class...> class Op, class... Args>
-struct detector<void_t<Op<Args...>>, Op, Args...> {
+struct detector<std::void_t<Op<Args...>>, Op, Args...> {
   using value_t = std::true_type;
 };
 } // end namespace detail
 
 /// Detects if a given trait holds for some set of arguments 'Args'.
 /// For example, the given trait could be used to detect if a given type
 /// has a copy assignment operator:
 ///   template<class T>