[libcxx] Only declare contents of threading API when _LIBCPP_HAS_THREAD_API_EXTERNAL is not defined.
[libcxx] Only declare contents of threading API when
_LIBCPP_HAS_THREAD_API_EXTERNAL is not defined.

When it is defined they will be declared by the
__external_threading header instead.

Differential revision: https://reviews.llvm.org/D66518

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@369537 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx