add constexpr

Author: vinay-deshmukh

libcxx/include/__utility/lazy_synth_three_way_comparator.h

@@ -96,10 +96,10 @@ struct __lazy_synth_three_way_comparator<_Comparator,
                                                             __has_default_three_way_comparator<_LHS, _RHS> >::value> > {
   // This lifetimebound annotation is technically incorrect, but other specializations actually capture the lifetime of
   // the comparator.
-  _LIBCPP_HIDE_FROM_ABI __lazy_synth_three_way_comparator(_LIBCPP_CTOR_LIFETIMEBOUND const _Comparator&) {}
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 __lazy_synth_three_way_comparator(_LIBCPP_CTOR_LIFETIMEBOUND const _Comparator&) {}
 
   // Same comment as above.
-  _LIBCPP_HIDE_FROM_ABI static __eager_compare_result
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 static __eager_compare_result
   operator()(_LIBCPP_LIFETIMEBOUND const _LHS& __lhs, _LIBCPP_LIFETIMEBOUND const _RHS& __rhs) {
     return __eager_compare_result(-__default_three_way_comparator<_LHS, _RHS>()(__lhs, __rhs));
   }