Split thread test into two parts. Mark one as XFAIL with ASAN.

The second part of the test checks that std::terminate is called when a running
thread is move assigned to. Calling std::terminate prevents some of the destructors
to be called and ASAN fires on this.



git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@222076 91177308-0d34-0410-b5e6-96231b3b80d8