Thanks for all the input!

After a second look at the standard, you are right.
It's not in the range of any of the universal-character-names.
That being said, it is allowed as an identifier by default in clang (as @tahonermann) mentioned.
It also works in current versions of clang if you use the UCN in the numeric constant (and leave the user-defined literal operator declaration with a $ sign).
As you stated, GCC also accepts this syntax.