Fix for long Xcode 15 build time after removing Representation concept

For some reason this new implementation of `RepresentationOf` was
causing long build times again in the Kalman fiter examples.  I'm not
sure why this is and if we should keep the old implemention only for
Xcode 15 or if we should revert this implementation change in general.

Author: rothmichaels

src/core/include/mp-units/framework/representation_concepts.h

@@ -340,11 +340,22 @@ concept SomeRepresentation =
 
 }  // namespace detail
 
+#ifdef MP_UNITS_XCODE15_HACKS
+MP_UNITS_EXPORT template<typename T, auto V>
+concept RepresentationOf =
+  detail::SomeRepresentation<T> &&
+  ((QuantitySpec<MP_UNITS_REMOVE_CONST(decltype(V))> &&
+    (detail::QuantityKindSpec<MP_UNITS_REMOVE_CONST(decltype(V))> || detail::IsOfCharacter<T, V.character>)) ||
+   (std::same_as<quantity_character, decltype(V)> && detail::IsOfCharacter<T, V>));
+
+#else
+
 MP_UNITS_EXPORT template<typename T, auto V>
 concept RepresentationOf =
   ((QuantitySpec<MP_UNITS_REMOVE_CONST(decltype(V))> &&
     ((detail::QuantityKindSpec<MP_UNITS_REMOVE_CONST(decltype(V))> && detail::SomeRepresentation<T>) ||
      detail::IsOfCharacter<T, V.character>)) ||
    (std::same_as<quantity_character, decltype(V)> && detail::IsOfCharacter<T, V>));
+#endif
 
 }  // namespace mp_units