[libc++] P2944R3: Constrained comparisions - the optional and reference_wrapper

Partially implements [P2944R3](https://wg21.link/P2944R3), which adds constrained comparisons to `optional` and `reference_wrapper`.

Generalizes the test helpers in `test_comparisons.h` to support constrained comparisons.

- [refwrap.comparisons](https://wg21.link/refwrap.comparisons)
- [optional.relops](https://wg21.link/optional.relops)
- [https://wg21.link/optional.comp.with.t](https://wg21.link/optional.comp.with.t)

Closes llvm#136767
Closes llvm#138233

Relates to: llvm#135759

Author: H-G-Hristov

libcxx/docs/Status/Cxx2cPapers.csv

@@ -59,7 +59,7 @@
 "`P2248R8 <https://wg21.link/P2248R8>`__","Enabling list-initialization for algorithms","2024-03 (Tokyo)","","",""
 "`P2810R4 <https://wg21.link/P2810R4>`__","``is_debugger_present`` ``is_replaceable``","2024-03 (Tokyo)","","",""
 "`P1068R11 <https://wg21.link/P1068R11>`__","Vector API for random number generation","2024-03 (Tokyo)","","",""
-"`P2944R3 <https://wg21.link/P2944R3>`__","Comparisons for ``reference_wrapper``","2024-03 (Tokyo)","|Partial|","","Implemented changes to ``reference_wrapper`` and ``pair``"
+"`P2944R3 <https://wg21.link/P2944R3>`__","Comparisons for ``reference_wrapper``","2024-03 (Tokyo)","|Partial|","","Not implemented ``tuple`` and ``variant``"
 "`P2642R6 <https://wg21.link/P2642R6>`__","Padded ``mdspan`` layouts","2024-03 (Tokyo)","","",""
 "`P3029R1 <https://wg21.link/P3029R1>`__","Better ``mdspan``'s CTAD","2024-03 (Tokyo)","|Complete|","19",""
 "","","","","",""

libcxx/include/__functional/reference_wrapper.h

@@ -11,12 +11,12 @@
 #define _LIBCPP___FUNCTIONAL_REFERENCE_WRAPPER_H
 
 #include <__compare/synth_three_way.h>
-#include <__concepts/boolean_testable.h>
 #include <__config>
 #include <__functional/weak_result_type.h>
 #include <__memory/addressof.h>
 #include <__type_traits/desugars_to.h>
 #include <__type_traits/enable_if.h>
+#include <__type_traits/is_core_convertible.h>
 #include <__type_traits/invoke.h>
 #include <__type_traits/is_const.h>
 #include <__type_traits/remove_cvref.h>
@@ -75,23 +75,23 @@ class reference_wrapper : public __weak_result_type<_Tp> {
 
   _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(reference_wrapper __x, reference_wrapper __y)
     requires requires {
-      { __x.get() == __y.get() } -> __boolean_testable;
+      { __x.get() == __y.get() } -> __core_convertible_to<bool>;
     }
   {
     return __x.get() == __y.get();
   }
 
   _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(reference_wrapper __x, const _Tp& __y)
     requires requires {
-      { __x.get() == __y } -> __boolean_testable;
+      { __x.get() == __y } -> __core_convertible_to<bool>;
     }
   {
     return __x.get() == __y;
   }
 
   _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(reference_wrapper __x, reference_wrapper<const _Tp> __y)
     requires(!is_const_v<_Tp>) && requires {
-      { __x.get() == __y.get() } -> __boolean_testable;
+      { __x.get() == __y.get() } -> __core_convertible_to<bool>;
     }
   {
     return __x.get() == __y.get();