LLVM supports TLS on Windows and we can use it from Clang

Patch by Martell Malone!

Differential Revision: http://reviews.llvm.org/D3421

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207470 91177308-0d34-0410-b5e6-96231b3b80d8