src/c-types-impl.lisp (make-or-intern-c-type): Pull out useful function.

src/c-types-impl.lisp (make-or-intern-c-type): Pull out useful function.

There's a recurring pattern whether a subtype is interned and using that
to decide whether to intern the derived type.  Pull it out into its own
function.  We'll want it more later; but even now it simplifies a couple
of call sites.