Make the ParentVector default to size 2.

As Jordan noted, it makes more sense when we store it in the ParentMap,
and doesn't really make a difference when we return it from getParents.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209530 91177308-0d34-0410-b5e6-96231b3b80d8