[XRay][compiler-rt] Reduce XRay log spam

This change makes XRay print the log file output only when the verbosity
level is higher than 0. It reduces the log spam in the default case when
we want XRay running silently, except when there are actual
fatal/serious errors.

We also update the documentation to show how to get the information
after the change to the default behaviour.

llvm-svn: 320550