Have a general mechanism for collecting code coverage

Code coverage support in Bazel is poor and one reason is the lack of a general mechanism for collecting the code coverage information.

As briefly described in [Bazel coverage design doc](https://docs.google.com/document/d/1-ZWHF-Q-qCKf19ik-t33ie58BkNurrYYzKR4OLtcilY/edit?usp=sharing), the next steps towards a more general solution are:
* Design and implement a `coverage_toolchain`
* Migrate Bazel builtin C++ code coverage collection to use the new mechanism.

Start by reading the linked Bazel coverage design doc, then inspect Bazel's existing builtin C++ code coverage collection. Define the coverage_toolchain mechanism and migrate the builtin C++ collection to it; completion requires both design goals to be implemented and validated.

Written by the indexing model from the issue text.