[XRay][compiler-rt] Use `SANITIZER_CXX_ABI_LIBRARY` for XRay unit tests

Summary:
Fix a TODO in CMake config for XRay tests to use the detected C++ ABI
library in the tests.

Also make the tests depend on the llvm-xray target when built in-tree.

Reviewers: kpw, eizan

Reviewed By: eizan

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D49358

llvm-svn: 337142