[libc++][ranges] LWG3618: Unnecessary iter_move for transform_view::iterator (#91809)

Summary:
## Introduction

This patch implements LWG3618: Unnecessary `iter_move` for
`transform_view::iterator`.

`transform_view`'s iterator currently specifies a customization point
for `iter_move`. This customization point does the same thing that the
default implementation would do, but its sole purpose is to ensure the
appropriate conditional `noexcept` specification.

## Reference

-
[[range.transform.iterator]](https://eel.is/c++draft/range.transform.iterator)
- [LWG3618](https://cplusplus.github.io/LWG/issue3618)

Test Plan: 

Reviewers: 

Subscribers: 

Tasks: 

Tags: 


Differential Revision: https://phabricator.intern.facebook.com/D60251361

Author: Xiaoyang Liu

libcxx/docs/Status/Cxx23Issues.csv

@@ -146,7 +146,7 @@
 "`3610 <https://wg21.link/LWG3610>`__","``iota_view::size`` sometimes rejects integer-class types","February 2022","","","|ranges|"
 "`3612 <https://wg21.link/LWG3612>`__","Inconsistent pointer alignment in ``std::format`` ","February 2022","|Complete|","14.0","|format|"
 "`3616 <https://wg21.link/LWG3616>`__","LWG 3498 seems to miss the non-member ``swap`` for ``basic_syncbuf`` ","February 2022","",""
-"`3618 <https://wg21.link/LWG3618>`__","Unnecessary ``iter_move`` for ``transform_view::iterator`` ","February 2022","","","|ranges|"
+"`3618 <https://wg21.link/LWG3618>`__","Unnecessary ``iter_move`` for ``transform_view::iterator`` ","February 2022","|Complete|","19.0","|ranges|"
 "`3619 <https://wg21.link/LWG3619>`__","Specification of ``vformat_to`` contains ill-formed ``formatted_size`` calls","February 2022","|Nothing to do|","","|format|"
 "`3621 <https://wg21.link/LWG3621>`__","Remove feature-test macro ``__cpp_lib_monadic_optional`` ","February 2022","|Complete|","15.0"
 "`3632 <https://wg21.link/LWG3632>`__","``unique_ptr`` ""Mandates: This constructor is not selected by class template argument deduction""","February 2022","|Nothing to do|",""

libcxx/include/__ranges/transform_view.h

@@ -326,13 +326,6 @@ class transform_view<_View, _Fn>::__iterator : public __transform_view_iterator_
   {
     return __x.__current_ - __y.__current_;
   }
-
-  _LIBCPP_HIDE_FROM_ABI friend constexpr decltype(auto) iter_move(const __iterator& __i) noexcept(noexcept(*__i)) {
-    if constexpr (is_lvalue_reference_v<decltype(*__i)>)
-      return std::move(*__i);
-    else
-      return *__i;
-  }
 };
 
 #  if _LIBCPP_STD_VER >= 23

libcxx/test/std/ranges/range.adaptors/range.transform/iterator/deref.pass.cpp

@@ -37,7 +37,7 @@ int main(int, char**) {
     using View = std::ranges::transform_view<MoveOnlyView, PlusOneNoexcept>;
     View transformView(MoveOnlyView{buff}, PlusOneNoexcept{});
     assert(*transformView.begin() == 1);
-    LIBCPP_ASSERT_NOEXCEPT(*std::declval<std::ranges::iterator_t<View>>());
+    ASSERT_NOEXCEPT(*std::declval<std::ranges::iterator_t<View>>());
     ASSERT_SAME_TYPE(int, decltype(*std::declval<View>().begin()));
   }
   {