It would be possible but I don't think we want to do that. Mangling for nested entities intentionally reflects the lexical structure, so that, for example, Demangle::getTypeForMangling() can easily find the declaration. If you mangle declarations with a where clause as if they were part of a fake constrained extension it would complicate code like that.
Also, we already mangle generic declarations with where clauses that reference outer parameters differently from constrained extension members, so introducing a special case here is even more error prone.