Merging r369537:

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r369537 | davidspickett | 2019-08-21 17:38:24 +0200 (Wed, 21 Aug 2019) | 7 lines

[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
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llvm-svn: 369639