[Sanitizer] Call Windows unwinder 'slow' and share StackTrace::Unwind across all platforms. No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@194193 91177308-0d34-0410-b5e6-96231b3b80d8
[Sanitizer] Presumable fix stack trace unwinding on Windows

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@194195 91177308-0d34-0410-b5e6-96231b3b80d8