[libc++] Mark more types as trivially relocatable (#89724)

Co-authored-by: Louis Dionne <ldionne.2@gmail.com>

libcxx/include/__split_buffer

@@ -24,12 +24,14 @@
 #include <__memory/pointer_traits.h>
 #include <__memory/swap_allocator.h>
 #include <__type_traits/add_lvalue_reference.h>
+#include <__type_traits/conditional.h>
 #include <__type_traits/enable_if.h>
 #include <__type_traits/integral_constant.h>
 #include <__type_traits/is_nothrow_assignable.h>
 #include <__type_traits/is_nothrow_constructible.h>
 #include <__type_traits/is_swappable.h>
 #include <__type_traits/is_trivially_destructible.h>
+#include <__type_traits/is_trivially_relocatable.h>
 #include <__type_traits/remove_reference.h>
 #include <__utility/forward.h>
 #include <__utility/move.h>
@@ -64,6 +66,15 @@ public:
   using iterator        = pointer;
   using const_iterator  = const_pointer;
 
+  // A __split_buffer contains the following members which may be trivially relocatable:
+  // - pointer: may be trivially relocatable, so it's checked
+  // - allocator_type: may be trivially relocatable, so it's checked
+  // __split_buffer doesn't have any self-references, so it's trivially relocatable if its members are.
+  using __trivially_relocatable = __conditional_t<
+      __libcpp_is_trivially_relocatable<pointer>::value && __libcpp_is_trivially_relocatable<allocator_type>::value,
+      __split_buffer,
+      void>;
+
   pointer __first_;
   pointer __begin_;
   pointer __end_;