Support: Remove MSVC 2013 workarounds in ThreadPool class.

I have confirmed that these are no longer needed with MSVC 2015.

Differential Revision: https://reviews.llvm.org/D34187

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@305347 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed