[XRay] [compiler-rt] make sure single threaded programs get traced too

Summary:
When the XRay user calls the API to finish writing the log, the thread
which is calling the API still hasn't finished and therefore won't get
its trace written.  Add a test for only the main thread to check this.

Reviewers: dberris

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38493

llvm-svn: 314875