[ADT] Move llvm::is_detected to STLForwardCompat.h (#158004)

llvm::is_detected forward-ported from C++20.  As such, it belongs to
STLForwardCompat.h.

Author: kazutakahirata

llvm/include/llvm/ADT/STLExtras.h

@@ -58,26 +58,6 @@ template <typename T> struct make_const_ref {
   using type = std::add_lvalue_reference_t<std::add_const_t<T>>;
 };
 
-namespace detail {
-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<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>
-///   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;
-
 /// 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>

llvm/include/llvm/ADT/STLForwardCompat.h

@@ -49,6 +49,26 @@ template <typename T>
 using type_identity_t // NOLINT(readability-identifier-naming)
     = typename llvm::type_identity<T>::type;
 
+namespace detail {
+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<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>
+///   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;
+
 //===----------------------------------------------------------------------===//
 //     Features from C++23
 //===----------------------------------------------------------------------===//