[libc++] Remove _LIBCPP_ABI_UNSTABLE

Previously, _LIBCPP_ABI_UNSTABLE would be used interchangeably with
_LIBCPP_ABI_VERSION >= 2. This was confusing and creating unnecessary
complexity.

This patch removes _LIBCPP_ABI_UNSTABLE -- instead, the LIBCXX_ABI_UNSTABLE
CMake option will result in the LIBCXX_ABI_VERSION being set to '2', the
current unstable ABI. As a result, in the code, we only have _LIBCPP_ABI_VERSION
to check in order to query the current ABI version.

As a fly-by, this also defines the ABI namespace during CMake configuration
to reduce complexity in __config. I believe it was previously done this
way because we used to try to use __config_site as seldom as possible.
Now that we always ship a __config_site, it doesn't really matter and
I think being explicit about how the library is configured in the __config_site
is actually a feature.

Differential Revision: https://reviews.llvm.org/D119173