Avoid lambda to work around clang assertion failure

Using a lambda in a requires clause can cause clang to assert, so
move to a named function instead.

Clang bug: llvm/llvm-project#64086

sus/num/integer_concepts.h

@@ -63,25 +63,28 @@ template <class T>
 concept PrimitiveInteger =
     UnsignedPrimitiveInteger<T> || SignedPrimitiveInteger<T>;
 
+namespace __private {
+// Workaround for https://github.com/llvm/llvm-project/issues/64086 which
+// prevents us from doing this as a lambda inside the requires clause.
+template <class T>
+void implicit_conversion(T) {}
+}  // namespace __private
+
 /// Enum types that are backed by an unsigned value, excluding `enum class`
 /// types.
 template <class T>
 concept UnsignedPrimitiveEnum =
     std::is_enum_v<T> && UnsignedPrimitiveInteger<std::underlying_type_t<T>> &&
     requires(T t) {
-      {
-        [](std::underlying_type_t<T>) {}(t)
-      };
+      { __private::implicit_conversion<std::underlying_type_t<T>>(t) };
     };
 
 /// Enum types that are backed by a signed value, excluding `enum class` types.
 template <class T>
 concept SignedPrimitiveEnum =
     std::is_enum_v<T> && SignedPrimitiveInteger<std::underlying_type_t<T>> &&
     requires(T t) {
-      {
-        [](std::underlying_type_t<T>) {}(t)
-      };
+      { __private::implicit_conversion<std::underlying_type_t<T>>(t) };
     };
 
 /// Enum types that are backed by a signed or unsigned value, excluding `enum