[asan] Make __asan_handle_no_return tolerate unregistered threads

Summary:
On Windows, thread injection by the kernel or other running processes is
a fairly common occurrence, so ASan should be resilient to it.  The
comments on GetCurrentThread() say that it can return null, so we
shouldn't be CHECK failing if it does.

Sending control-C is one way to get the kernel to inject a thread into
your process, so I wrote a test around it.

Reviewers: llvm-commits

Subscribers: samsonov

Differential Revision: http://reviews.llvm.org/D11426

llvm-svn: 242948