[ox/std] Add std::is_constant_evaluated()

(synced from 180cac5cc6)

src/ox/std/typetraits.hpp

@@ -21,6 +21,10 @@ namespace std {
 template<typename T>
 constexpr bool is_union_v = __is_union(T);
 
+constexpr bool is_constant_evaluated() noexcept {
+	return __builtin_is_constant_evaluated();
+}
+
 }
 
 #endif