[Sema] Use the right FoldingSet.

We were doing FindNodeOrInsertPos on SubstTemplateTypeParmPackTypes, so
we should presumably be inserting into SubstTemplateTypeParmPackTypes.

Looks like the FoldingSet API can be tweaked a bit so that we can catch
things like this at compile-time. I'll look into that shortly.

I'm unsure of how to test this; suggestions welcome.

Thanks to Vladimir Voskresensky for bringing this up!

llvm-svn: 305207