Hide the specializations folding sets of ClassTemplateDecl as an implementation detail (InsertPos

leaks though) and add methods to its interface for adding/finding specializations.

Simplifies its users a bit and we no longer need to replace specializations in the folding set with
their redeclarations. We just return the most recent redeclarations.

As a bonus, it fixes http://llvm.org/PR7670.

llvm-svn: 108832