[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.

Author: philnik777

libcxx/include/CMakeLists.txt

@@ -835,7 +835,6 @@ set(files
   __type_traits/is_member_pointer.h
   __type_traits/is_nothrow_assignable.h
   __type_traits/is_nothrow_constructible.h
-  __type_traits/is_nothrow_convertible.h
   __type_traits/is_nothrow_destructible.h
   __type_traits/is_null_pointer.h
   __type_traits/is_object.h

libcxx/include/__type_traits/is_convertible.h

@@ -26,6 +26,16 @@ template <class _From, class _To>
 _LIBCPP_NO_SPECIALIZATIONS inline constexpr bool is_convertible_v = __is_convertible(_From, _To);
 #endif
 
+#if _LIBCPP_STD_VER >= 20
+
+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);
+
+#endif // _LIBCPP_STD_VER >= 20
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___TYPE_TRAITS_IS_CONVERTIBLE_H

libcxx/include/__type_traits/is_nothrow_convertible.h

@@ -232,10 +232,6 @@ module std_core [system] {
       header "__type_traits/is_nothrow_constructible.h"
       export std_core.type_traits.integral_constant
     }
-    module is_nothrow_convertible {
-      header "__type_traits/is_nothrow_convertible.h"
-      export std_core.type_traits.integral_constant
-    }
     module is_nothrow_destructible {
       header "__type_traits/is_nothrow_destructible.h"
       export std_core.type_traits.integral_constant

libcxx/include/module.modulemap.in

@@ -549,7 +549,6 @@ namespace std
 #    include <__type_traits/common_reference.h>
 #    include <__type_traits/is_bounded_array.h>
 #    include <__type_traits/is_constant_evaluated.h>
-#    include <__type_traits/is_nothrow_convertible.h>
 #    include <__type_traits/is_unbounded_array.h>
 #    include <__type_traits/type_identity.h>
 #    include <__type_traits/unwrap_ref.h>