[PATCH] D28316: [libc++] Cleanup and document <__threading_support>

[Eric Fiselier via Phabricator via cfe-commits]

EricWF added a comment.

@compnerd Just waiting on @rmaprath before committing. This is a breaking change for his use case since this patch changes the semantics of `_LIBCPP_HAS_THREAD_API_EXTERNAL` and `-DLIBCXX_HAS_EXTERNAL_THREAD_API`.


https://reviews.llvm.org/D28316