[MS ABI] __declspec(thread) behaves like thread_local in MSVC 2015

MSVC 2015 changed __declspec(thread) to make it behave like C++11's
thread_local keyword instead of acting similarly to __thread.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@237337 91177308-0d34-0410-b5e6-96231b3b80d8