[winasan] Unpoison stack memory when threads exit (redux)

This is a second attempt at r342652 using a TLS callback instead of an
interceptor.

In long-running builds we've seen some ASan complaints during thread creation
that we suspect are due to leftover poisoning from previous threads whose
stacks occupied that memory. This patch adds a callback that unpoisons the
stack memory when a thread exits.

Differential Revision: https://reviews.llvm.org/D58641

llvm-svn: 354836