Make the ParentVector default to size 2.
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: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@209530 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang