Support decomposing types that only compare with literal 0

This is primarily done to support new `std::*_ordering` types,
but the refactoring also supports any other type with this
property.

The compilation overhead is surprisingly low. Testing it with
clang on a Linux machine, compiling our SelfTest project takes
only 2-3% longer with these changes than it takes otherwise.

Closes #2555

tests/SelfTest/helpers/type_with_lit_0_comparisons.hpp

@@ -0,0 +1,44 @@
+
+//              Copyright Catch2 Authors
+// Distributed under the Boost Software License, Version 1.0.
+//   (See accompanying file LICENSE.txt or copy at
+//        https://www.boost.org/LICENSE_1_0.txt)
+
+// SPDX-License-Identifier: BSL-1.0
+
+#ifndef CATCH_TEST_HELPERS_TYPE_WITH_LIT_0_COMPARISONS_HPP_INCLUDED
+#define CATCH_TEST_HELPERS_TYPE_WITH_LIT_0_COMPARISONS_HPP_INCLUDED
+
+#include <type_traits>
+
+// Should only be constructible from literal 0.
+// Used by `TypeWithLit0Comparisons` for testing comparison
+// ops that only work with literal zero, the way std::*orderings do
+struct ZeroLiteralDetector {
+    constexpr ZeroLiteralDetector( ZeroLiteralDetector* ) noexcept {}
+
+    template <typename T,
+              typename = std::enable_if_t<!std::is_same<T, int>::value>>
+    constexpr ZeroLiteralDetector( T ) = delete;
+};
+
+struct TypeWithLit0Comparisons {
+#define DEFINE_COMP_OP( op )                                                  \
+    friend bool operator op( TypeWithLit0Comparisons, ZeroLiteralDetector ) { \
+        return true;                                                          \
+    }                                                                         \
+    friend bool operator op( ZeroLiteralDetector, TypeWithLit0Comparisons ) { \
+        return false;                                                         \
+    }
+
+    DEFINE_COMP_OP( < )
+    DEFINE_COMP_OP( <= )
+    DEFINE_COMP_OP( > )
+    DEFINE_COMP_OP( >= )
+    DEFINE_COMP_OP( == )
+    DEFINE_COMP_OP( != )
+
+#undef DEFINE_COMP_OP
+};
+
+#endif // CATCH_TEST_HELPERS_TYPE_WITH_LIT_0_COMPARISONS_HPP_INCLUDED