[XRay][compiler-rt] Make unit tests depend on implementation files

Summary:
This change makes changes to XRay implementation files trigger re-builds
of the unit tests. Prior to this change, the unit tests were not built
and run properly if the implementation files were changed during the
development process. This change forces the dependency on all files in
the XRay include and lib hosted files in compiler-rt.

Caveat is, that new files added to the director(ies) will need a re-run
of CMake to re-generate the fileset.

We think this is an OK compromise, since adding new files may
necessitate editing (or adding) new unit tests. It's also less likely
that we're adding new files without updating the CMake configuration to
include the functionality in the XRay runtime implementation anyway.

Reviewers: pelikan, kpw, nglevin

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D44080

llvm-svn: 326842