Previously we were making them into skolem TcTyVars,
which is wrong for the output of the type checker, which
no TcTyVars should surive.

See Note [Zonking the LHS of a RULE] in TcHsSyn

This was flushed out by the new IfaceTcTyVar thing;
I found some more TcTyVars that were being serialised into
an interface file, which is wrong wrong wrong.