[XRay][compiler-rt] Enable the XRay compiler-rt unit tests.

Summary:
Before this change we seemed to not be running the unit tests, and therefore we
set out to run them. In the process of making this happen we found a divergence
between the implementation and the tests.

This includes changes to both the CMake files as well as the implementation and
headers of the XRay runtime. We've also updated documentation on the changed
functions.

Reviewers: kpw, eizan

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D37290

llvm-svn: 312202