Introduce a sequence number into class template partial

Introduce a sequence number into class template partial
specializations, which keeps track of the order in which they were
originally declared. We use this number so that we can always walk the
list of partial specializations in a predictable order during matching
or template instantiation. This also fixes a failure in Boost.Proto,
where SourceManager::isBeforeInTranslationUnit was behaving
poorly in inconsistent ways.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@102693 91177308-0d34-0410-b5e6-96231b3b80d8