[libc++abi] Bump the libc++abi version and add it to the post-release list

[ldionne]

Heads up @tstellar about updating this in the future -- I'm not sure how you keep track of those, but I added it to our post-release TODO list in case that helps. If you have another preferred way of tracking these things, please let me know and I'll add it there instead.

[tstellar]

In D119947#3326417, @ldionne wrote:

Heads up @tstellar about updating this in the future -- I'm not sure how you keep track of those, but I added it to our post-release TODO list in case that helps. If you have another preferred way of tracking these things, please let me know and I'll add it there instead.

I usually just look at the version bump for the previous release to see what files I need to change. I can try to remember to do this one too, you could also just ping me a few days before the release branch is created to help me remember.

I was also wondering if there is some way to pull this value from the versions in the CMake files?

libcxx/docs/Contributing.rst

 consists of frequent libc++ contributors with a good understanding of the project's
 guidelines -- if you would like to be added to it, please reach out on Discord.
 
 Post-release check list
 =======================
 
 After branching for an LLVM release:
 
-1. Update ``_LIBCPP_VERSION`` in ``include/__config``
-2. Update the ``include/__libcpp_version`` file
-3. Update the version number in ``docs/conf.py``
+1. Update ``_LIBCPP_VERSION`` in ``libcxx/include/__config``
+2. Update the ``libcxx/include/__libcpp_version`` file
+3. Update the version number in ``libcxx/docs/conf.py``
+4. Update ``_LIBCPPABI_VERSION`` in ``libcxxabi/include/cxxabi.h``
 
 Exporting new symbols from the library
 ======================================
 
 When exporting new symbols from libc++, you must update the ABI lists located in ``lib/abi``.
 To test whether the lists are up-to-date, please run the target ``check-cxx-abilist``.
 To regenerate the lists, use the target ``generate-cxx-abilist``.
 The ABI lists must be updated for all supported platforms; currently Linux and

libcxxabi/include/cxxabi.h

  *       https://itanium-cxx-abi.github.io/cxx-abi/
  */
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <__cxxabi_config.h>
 
-#define _LIBCPPABI_VERSION 1002
+#define _LIBCPPABI_VERSION 15000
 #define _LIBCXXABI_NORETURN  __attribute__((noreturn))
 #define _LIBCXXABI_ALWAYS_COLD __attribute__((cold))
 
 #ifdef __cplusplus
 
 namespace std {
 #if defined(_WIN32)
 class _LIBCXXABI_TYPE_VIS type_info; // forward declaration