Remove SequenceNumber from class/variable template partial specializations.
This was only used to ensure that the traversal order was the same as the
insertion order, but that guarantee was already being provided by the use
of a FoldingSetVector.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189075 91177308-0d34-0410-b5e6-96231b3b80d8