[PATCH] Add a new routine to libc++abi to get the number of uncaught exceptions

[Eric Fiselier]

I think that this patch should bump the value of `_LIBCPPAPI_VERSION`. libc++ is going to need to know when `libc++abi` supports `__cxa_uncaught_exceptions()` and when it doesn't in order to provide `std::uncaught_exceptions()`. Using the value of `_LIBCPPABI_VERSION` seems like an appropriate approach.

Other than that and @jroelofs request for tests this patch LGTM.


http://reviews.llvm.org/D10067

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