[libc++] Add missing std::is_virtual_base_of to type_traits.inc (#107009)

std::is_virtual_base_of was implemented in #105847

Author: H-G-Hristov

libcxx/modules/std/type_traits.inc

@@ -114,6 +114,9 @@ export namespace std {
 
   // [meta.rel], type relations
   using std::is_base_of;
+#if _LIBCPP_STD_VER >= 26 && __has_builtin(__builtin_is_virtual_base_of)
+  using std::is_virtual_base_of;
+#endif
   using std::is_convertible;
   // using std::is_layout_compatible;
   using std::is_nothrow_convertible;
@@ -287,6 +290,9 @@ export namespace std {
 
   // [meta.rel], type relations
   using std::is_base_of_v;
+#if _LIBCPP_STD_VER >= 26 && __has_builtin(__builtin_is_virtual_base_of)
+  using std::is_virtual_base_of_v;
+#endif
   using std::is_convertible_v;
   using std::is_invocable_r_v;
   using std::is_invocable_v;