Issue 8847 - voldemort + inout confuses "is"

http://d.puremagic.com/issues/show_bug.cgi?id=8847

By this pull, all mangled result of inner aggregate types in auto functions are changed.

As I wrote [in bugzilla](http://d.puremagic.com/issues/show_bug.cgi?id=8847#c5), Voldemort types cannot be mangled correctly with current mangling rule. I think that a special exception is necessary there.