Import CodeCoverage.cmake from bilke/cmake-modules

This BSD-licensed third-party [coverage module][1] for CMake will help restore
the coverage capability that was removed in fc6572c.

[1]: https://github.com/bilke/cmake-modules/blob/d98828f54f6974717798e63195cfbf08fe2daad0/CodeCoverage.cmake

Author: kulp