Add C++ test coverage (#4812)
* Add C++ test coverage
* Update CMakeLists.txt
* Update setup.py
* Install lcov in circleci
* Fix lcov installation
* Move lcov command out of CMake file
* Formatting
* Move get_coverage.sh around
* Fix filepath
* Add some debugging
* Try full path
* Make coverage file executable
* Install LLVM
* Add -y to install
* Fix apt-get
* Try different llvm installation
* Remove extra clang installation
* echo path before llvm-cov
* Try another llvm installation approach
* Add debugging for llvm-cov installation
* Add llvm-8/bin to path
* Prepend PATH in run tests
* Test upload step
* Move coverage commands to CircleCI
* Build xla with debug symbols
* Update common.sh
* Use chmod on coverage script
* Fix copy
* Try different approach to moving file
* Cleanup
* Remove unnecessary ls
* Remove USE_COVERAGE from cpp/run_tests.sh
* Fix line at end of file
* Remove extra space
