Libc++ ABI stability

Libc++ aims to preserve stable ABI to avoid subtle bugs when code built to the old ABI is linked with the code build to the new ABI. At the same time, libc++ allows ABI-breaking improvements and bugfixes for the scenarios when ABI change is not a issue.

To support both cases, libc++ allows specifying the ABI version at the build time. The version is defined with a cmake option LIBCXX_ABI_VERSION. Another option LIBCXX_ABI_UNSTABLE can be used to include all present ABI breaking features. These options translate into C++ macro definitions _LIBCPP_ABI_VERSION, _LIBCPP_ABI_UNSTABLE.

Any ABI-changing feature is placed under it's own macro, _LIBCPP_ABI_XXX, which is enabled based on the value of _LIBCPP_ABI_VERSION. _LIBCPP_ABI_UNSTABLE, if set, enables all features at once.