[libcxx] Add `__split_buffer::__swap_layouts` (#180102)

This commit simplifies the cumbersome process of swapping the respective
layout members for `__split_buffer` and `vector`.

NOKEYCHECK=True
GitOrigin-RevId: 65f39a16abf651008185839459fb330258800a62

include/__split_buffer

diff --git a/include/__split_buffer b/include/__split_buffer
index 89c398e..27ec8b7 100644
--- a/include/__split_buffer
+++ b/include/__split_buffer
@@ -188,6 +188,14 @@
     __back_cap_  = __other.__back_cap_;
   }
 
+  _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI void
+  __swap_layouts(pointer& __begin, pointer& __end, pointer& __back_capacity) {
+    using std::swap;
+    swap(__begin_, __begin);
+    swap(__end_, __end);
+    swap(__back_cap_, __back_capacity);
+  }
+
 private:
   pointer __front_cap_ = nullptr;
   pointer __begin_     = nullptr;
@@ -336,6 +344,14 @@
     __size_      = __other.__size_;
   }
 
+  _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI void
+  __swap_layouts(pointer& __begin, size_type& __size, size_type& __capacity) {
+    using std::swap;
+    swap(__begin_, __begin);
+    swap(__size_, __size);
+    swap(__cap_, __capacity);
+  }
+
 private:
   pointer __front_cap_ = nullptr;
   pointer __begin_     = nullptr;
@@ -447,6 +463,7 @@
   using __base_type::__set_data;
   using __base_type::__set_sentinel;
   using __base_type::__set_valid_range;
+  using __base_type::__swap_layouts;
 
   using typename __base_type::__alloc_traits;
   using typename __base_type::allocator_type;

include/__vector/vector.h

diff --git a/include/__vector/vector.h b/include/__vector/vector.h
index 181230d..22b4f70 100644
--- a/include/__vector/vector.h
+++ b/include/__vector/vector.h
@@ -820,22 +820,9 @@
     return __p;
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI void __swap_layouts(_SplitBuffer& __sb) {
-    auto __vector_begin    = __begin_;
-    auto __vector_sentinel = __end_;
-    auto __vector_cap      = __cap_;
-
-    auto __sb_begin    = __sb.begin();
-    auto __sb_sentinel = __sb.__raw_sentinel();
-    auto __sb_cap      = __sb.__raw_capacity();
-
-    // TODO: replace with __set_valid_range and __set_capacity when vector supports it.
-    __begin_ = __sb_begin;
-    __end_   = __sb_sentinel;
-    __cap_   = __sb_cap;
-
-    __sb.__set_valid_range(__vector_begin, __vector_sentinel);
-    __sb.__set_capacity(__vector_cap);
+  _LIBCPP_NODEBUG _LIBCPP_ALWAYS_INLINE _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI void
+  __swap_layouts(__split_buffer<_Tp, allocator_type>& __sb) {
+    __sb.__swap_layouts(__begin_, __end_, __cap_);
   }
 };