Fix deserializing of class template partial specializations. Assign sequence

Fix deserializing of class template partial specializations. Assign sequence
numbers as we deserialize class template partial specializations. We can't
assume that the old sequence numbers will work.

The sequence numbers are still deterministic, but are now a lot less
predictable for class template partial specializations in modules/PCH.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184811 91177308-0d34-0410-b5e6-96231b3b80d8