For some ancient reason, TcType.isStringTy was treating newtypes as
transparent, which is quite consistent with isIntTy, isBoolTy etc.  
(I think the reason is that isStringTy was written to go with isFFIDotNetTy,
which deals in representation types.)

Anyway, this inconsistency is Utterly Wrong when called from 
Inst.shortCutStringLit, and that made tc224 fail.  I can't think how
it ever succeeded.  Maybe it never did!

Anyway this fixes it. It may be that .NET FFI calls are not quite as
permissive, but they are almost certainly broken in more serious ways,
so I'm going to jump that bridge if we come to it.