[libc++] Fix expression-equivalence for mem_fn

Previously, SFINAE constraints and exception specification propagation
were missing in the return type of libc++'s `std::mem_fn`. The
requirements on expression-equivalence (or even plain "equivalent" in
pre-C++20 specification) in [func.memfn] are actually requiring them.

This PR adds the missed stuffs.

Drive-by changes:
- removing no longer used `__invoke_return`,
- updating synopsis comments in several files, and
- merging several test files for `mem_fn` into one.

libcxx/include/__functional/mem_fn.h

@@ -36,10 +36,8 @@ class __mem_fn : public __weak_result_type<_Tp> {
 
   // invoke
   template <class... _ArgTypes>
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
-
-  typename __invoke_return<type, _ArgTypes...>::type
-  operator()(_ArgTypes&&... __args) const {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 typename __invoke_of<const _Tp&, _ArgTypes...>::type
+  operator()(_ArgTypes&&... __args) const _NOEXCEPT_(__nothrow_invokable<const _Tp&, _ArgTypes...>::value) {
     return std::__invoke(__f_, std::forward<_ArgTypes>(__args)...);
   }
 };

libcxx/include/__functional/weak_result_type.h

@@ -221,11 +221,6 @@ struct __weak_result_type<_Rp (_Cp::*)(_A1, _A2, _A3...) const volatile> {
 #endif
 };
 
-template <class _Tp, class... _Args>
-struct __invoke_return {
-  typedef decltype(std::__invoke(std::declval<_Tp>(), std::declval<_Args>()...)) type;
-};
-
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___FUNCTIONAL_WEAK_RESULT_TYPE_H

libcxx/include/functional

@@ -395,7 +395,7 @@ const_mem_fun_ref_t<S,T>    mem_fun_ref(S (T::*f)() const);
 template <class S, class T, class A>
 const_mem_fun1_ref_t<S,T,A> mem_fun_ref(S (T::*f)(A) const);                    // deprecated in C++11, removed in C++17
 
-template<class R, class T> constexpr unspecified mem_fn(R T::*);                // constexpr in C++20
+template<class R, class T> constexpr unspecified mem_fn(R T::*) noexcept;       // constexpr in C++20
 
 class bad_function_call
     : public exception