[ADT] Remove `is_invocable` from `STLExtras.h`

[jloser]

Summary

As a follow-up of https://reviews.llvm.org/D132318, now that the callers have
been adjusted to use std::is_invocable, remove llvm::is_invocable and its
tests.

[jloser]

Rebase to make sure CI failure is unrelated

[dblaikie]

great :)

llvm/include/llvm/ADT/STLExtras.h

 /// has a copy assignment operator:
 ///   template<class T>
 ///   using has_copy_assign_t = decltype(std::declval<T&>()
 ///                                                 = std::declval<const T&>());
 ///   bool fooHasCopyAssign = is_detected<has_copy_assign_t, FooClass>::value;
 template <template <class...> class Op, class... Args>
 using is_detected = typename detail::detector<void, Op, Args...>::value_t;
 
-namespace detail {
-template <typename Callable, typename... Args>
-using is_invocable =
-    decltype(std::declval<Callable &>()(std::declval<Args>()...));
-} // namespace detail
-
-/// Check if a Callable type can be invoked with the given set of arg types.
-template <typename Callable, typename... Args>
-using is_invocable = is_detected<detail::is_invocable, Callable, Args...>;
-
 /// This class provides various trait information about a callable object.
 ///   * To access the number of arguments: Traits::num_args
 ///   * To access the type of an argument: Traits::arg_t<Index>
 ///   * To access the type of the result:  Traits::result_t
 template <typename T, bool isClass = std::is_class<T>::value>
 struct function_traits : public function_traits<decltype(&T::operator())> {};
 
 /// Overload for class function types.

llvm/unittests/ADT/TypeTraitsTest.cpp

 
 template <class T> using has_foo_method_t = decltype(std::declval<T &>().foo());
 
 static_assert(is_detected<has_foo_method_t, HasFooMethod>::value,
               "expected foo method to be detected");
 static_assert(!is_detected<has_foo_method_t, NoFooMethod>::value,
               "expected no foo method to be detected");
 } // end anonymous namespace
-
-//===----------------------------------------------------------------------===//
-// is_invocable
-//===----------------------------------------------------------------------===//
-
-void invocable_fn(int);
-
-static_assert(is_invocable<decltype(invocable_fn), int>::value,
-              "expected function to be invocable");
-static_assert(!is_invocable<decltype(invocable_fn), void *>::value,
-              "expected function not to be invocable");
-static_assert(!is_invocable<decltype(invocable_fn), int, int>::value,
-              "expected function not to be invocable");