[XRay] Clean up XRay build configuration

Summary:
This change spans both LLVM and compiler-rt, where we do the following:

- Add XRay to the LLVMBuild system, to allow for distributing the XRay
  trace loading library along with the LLVM distributions.

- Use `llvm-config` better in the compiler-rt XRay implementation, to
  depend on the potentially already-distributed LLVM XRay library.

While this is tested with the standalone compiler-rt build, it does
require that the LLVMXRay library (and LLVMSupport as well) are
available during the build. In case the static libraries are available,
the unit tests will build and work fine. We're still having issues with
attempting to use a shared library version of the LLVMXRay library since
the shared library might not be accessible from the standard shared
library lookup paths.

The larger change here is the inclusion of the LLVMXRay library in the
distribution, which allows for building tools around the XRay traces and
profiles that the XRay runtime already generates.

Reviewers: echristo, beanz

Subscribers: mgorny, hiraditya, mboerger, llvm-commits

Differential Revision: https://reviews.llvm.org/D52349

llvm-svn: 342859