[libc++][NFC] Reduce use of __add_lvalue_reference_t (#112497)

Currently, the occurrences of `__add_lvalue_reference_t` in
`__split_buffer` and `__assoc_state` are probably meaningless.

* In `__split_buffer`, the `__alloc_ref` and `__alloc_const_ref` member
  typedefs are no longer used.
* In `__assoc_state`, we should simply use `_Rp&`, which must be
  well-formed since it's already required that `sizeof(_Rp)` is
  well-formed.

This PR removes the meaningless usages. The remaining occurrences in
`shared_ptr`, `unique_ptr`, and several type traits are meaningful.

Author: frederick-vs-ja

libcxx/include/__split_buffer

@@ -80,9 +80,6 @@ public:
   pointer __end_;
   _LIBCPP_COMPRESSED_PAIR(pointer, __end_cap_, allocator_type, __alloc_);
 
-  using __alloc_ref       = __add_lvalue_reference_t<allocator_type>;
-  using __alloc_const_ref = __add_lvalue_reference_t<allocator_type>;
-
   __split_buffer(const __split_buffer&)            = delete;
   __split_buffer& operator=(const __split_buffer&) = delete;
 

libcxx/include/future

@@ -594,7 +594,7 @@ public:
   _LIBCPP_HIDE_FROM_ABI void set_value_at_thread_exit(_Arg&& __arg);
 
   _LIBCPP_HIDE_FROM_ABI _Rp move();
-  _LIBCPP_HIDE_FROM_ABI __add_lvalue_reference_t<_Rp> copy();
+  _LIBCPP_HIDE_FROM_ABI _Rp& copy();
 };
 
 template <class _Rp>
@@ -636,7 +636,7 @@ _Rp __assoc_state<_Rp>::move() {
 }
 
 template <class _Rp>
-__add_lvalue_reference_t<_Rp> __assoc_state<_Rp>::copy() {
+_Rp& __assoc_state<_Rp>::copy() {
   unique_lock<mutex> __lk(this->__mut_);
   this->__sub_wait(__lk);
   if (this->__exception_ != nullptr)