libcxx: Move Windows threading support into a .cpp file.

This allows us to avoid polluting the namespace of users of <thread>
with the definitions in windows.h.

Differential Revision: https://reviews.llvm.org/D42214

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@323169 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed