Write references to Native format in NativeReferenceIvarsV2.

Summary:
NativeReferenceIvarsV1 cannot handle more than 65535 relocation targets
because the type of targetIndex is uint16_t. Because reasonable size of
executable can easily have more than 65535 relocation targets, the format is
far from being practical. The existence of RoundTripNativePass worsen the
sitatuion -- all linking results go through the pass to get converted to
Native and read it back. That means the LLD cannot link program having many
relocations even if you do not explicitly use Native format. I believe we
cannot self-host the LLD on Unix anymore because of the issue.

Need to migrate to V2.

Reviewers: kledzik, Bigcheese, shankarke

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2217