patches: Backport upstream fix for __is_convertible

Replicate upstream commit 484e64f7e7b2c0494d7b2dbfdd528bcd707ee652.
libcxx previously used the `__is_convertible_to` builtin which wasn't
supported by GCC; this patch adds support for `__is_convertible` which
is available as of GCC 13.

Signed-off-by: Andrei Tatar <[email protected]>
Reviewed-by: Stefan Jumarea <[email protected]>
Reviewed-by: Maria Sfiraiala <[email protected]>
Reviewed-by: Razvan Deaconescu <[email protected]>
Approved-by: Eduard Vintilă <[email protected]>
Tested-by: Unikraft CI <[email protected]>
GitHub-Closes: unikraft#28

Author: andreittr

patches/0001-Use-__is_convertible-built-in-when-available.patch

@@ -0,0 +1,24 @@
+diff --git a/libcxx/include/type_traits b/libcxx/include/type_traits
+index 3391999675..ab126d000d 100644
+--- a/include/type_traits
++++ b/include/type_traits
+@@ -1659,11 +1659,18 @@ struct __is_core_convertible<_Tp, _Up, decltype(
+ 
+ // is_convertible
+ 
+-#if __has_feature(is_convertible_to) && !defined(_LIBCPP_USE_IS_CONVERTIBLE_FALLBACK)
++#if __has_builtin(__is_convertible) && !defined(_LIBCPP_USE_IS_CONVERTIBLE_FALLBACK)
++
++template <class _T1, class _T2>
++struct _LIBCPP_TEMPLATE_VIS is_convertible : public integral_constant<bool, __is_convertible(_T1, _T2)> {};
++
++#elif __has_feature(is_convertible_to) && !defined(_LIBCPP_USE_IS_CONVERTIBLE_FALLBACK)
+ 
+ template <class _T1, class _T2> struct _LIBCPP_TEMPLATE_VIS is_convertible
+     : public integral_constant<bool, __is_convertible_to(_T1, _T2)> {};
+ 
++// TODO: Remove this fallback when GCC < 13 support is no longer required.
++// GCC 13 has the __is_convertible built-in.
+ #else  // __has_feature(is_convertible_to)
+ 
+ namespace __is_convertible_imp