Replace ad-hoc tracking of pattern for an instantiated class-scope

Replace ad-hoc tracking of pattern for an instantiated class-scope
explicit function specialization with the MemberSpecializationInfo used
everywhere else.

Not NFC: the ad-hoc pattern tracking was not being serialized /
deserialized properly. That's fixed here.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@359747 91177308-0d34-0410-b5e6-96231b3b80d8