[PATCH] Introduce _LIBCPP_ABI_UNSTABLE to libc++

[Chandler Carruth]

FYI, as I commented on the original discussion, I'd personally like to see this have some versioning scheme before it goes in rather than a binary "unstable". Or some plan for getting there.

Otherwise this looks good as a first step.


http://reviews.llvm.org/D6712

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