Hide the specializations folding sets of ClassTemplateDecl as an implementation detai...

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@108832 91177308-0d34-0410-b5e6-96231b3b80d8