Fix libfmt errors from not finding enum formatter

Recent versions of libfmt have become more strict and require
`enum` types to be formattable:

  static assertion failed due to requirement 'formattable': Cannot format an argument. To make type T formattable provide a formatter<T> specialization: https://fmt.dev/latest/api.html#udt

This is a quick fix to simply use the underlying type.

folly/futures/detail/Core.cpp

@@ -29,8 +29,10 @@ namespace {
 
 template <class Enum>
 void terminate_unexpected_state(fmt::string_view context, Enum state) {
-  terminate_with<std::logic_error>(
-      fmt::format("{} unexpected state: {}", context, state));
+  terminate_with<std::logic_error>(fmt::format(
+      "{} unexpected state: {}",
+      context,
+      static_cast<std::underlying_type_t<Enum>>(state)));
 }
 
 } // namespace