[Sema] Make SpecialMemberDecl a PointerIntPair so we can stash it in a SmallPtrSet.

We have enough spare bits in the alignment of CXXRecordDecl. No
functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@312186 91177308-0d34-0410-b5e6-96231b3b80d8