Introduce bit_cast_constexpr

Author: axic

lib/fizzy/cxx20/bit.hpp

@@ -20,6 +20,8 @@ namespace fizzy
 using std::bit_cast;
 }  // namespace fizzy
 
+#define bit_cast_constexpr constexpr
+
 #else
 
 namespace fizzy
@@ -41,6 +43,8 @@ template <class To, class From>
 }
 }  // namespace fizzy
 
+#define bit_cast_constexpr
+
 #endif /* __cpp_lib_bit_cast */
 
 #ifdef __cpp_lib_bitops