libcxx-compat: revert llvmorg-21-init-10154-ge43e8ec7afbd:

[libc++] Remove dead implementation of is_nothrow_convertible and merge the remaining code into is_convertible.h (#137717)

  We can use the `__is_nothrow_convertible` builtin unconditionally now,
  which makes the implementation very simple, so there isn't much of a
  need to keep a separate header around.

This is part of making libc++ 21 build with clang 18.

PR:		292067
MFC after:	1 month

(cherry picked from commit 035396ceda)

contrib/llvm-project/libcxx/include/__type_traits/is_convertible.h

@@ -10,6 +10,9 @@
 #define _LIBCPP___TYPE_TRAITS_IS_CONVERTIBLE_H
 
 #include <__config>
+#include <__type_traits/conjunction.h>
+#include <__type_traits/disjunction.h>
+#include <__type_traits/lazy.h>
 #include <__type_traits/integral_constant.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -28,12 +31,35 @@ _LIBCPP_NO_SPECIALIZATIONS inline constexpr bool is_convertible_v = __is_convert
 
 #if _LIBCPP_STD_VER >= 20
 
+#  if __has_builtin(__is_nothrow_convertible)
+
 template <class _Tp, class _Up>
 struct _LIBCPP_NO_SPECIALIZATIONS is_nothrow_convertible : bool_constant<__is_nothrow_convertible(_Tp, _Up)> {};
 
 template <class _Tp, class _Up>
 _LIBCPP_NO_SPECIALIZATIONS inline constexpr bool is_nothrow_convertible_v = __is_nothrow_convertible(_Tp, _Up);
 
+#  else // __has_builtin(__is_nothrow_convertible)
+
+template <typename _Tp>
+void __test_noexcept(_Tp) noexcept;
+
+template <typename _Fm, typename _To>
+bool_constant<noexcept(std::__test_noexcept<_To>(std::declval<_Fm>()))> __is_nothrow_convertible_test();
+
+template <typename _Fm, typename _To>
+struct __is_nothrow_convertible_helper : decltype(std::__is_nothrow_convertible_test<_Fm, _To>()) {};
+
+template <typename _Fm, typename _To>
+struct is_nothrow_convertible
+    : _Or<_And<is_void<_To>, is_void<_Fm>>,
+          _Lazy<_And, is_convertible<_Fm, _To>, __is_nothrow_convertible_helper<_Fm, _To> > >::type {};
+
+template <typename _Fm, typename _To>
+inline constexpr bool is_nothrow_convertible_v = is_nothrow_convertible<_Fm, _To>::value;
+
+#  endif // __has_builtin(__is_nothrow_convertible)
+
 #endif // _LIBCPP_STD_VER >= 20
 
 _LIBCPP_END_NAMESPACE_STD