Clang source-based code coverage with fuzz test

It seems clang supports source-based [code coverage tool](https://clang.llvm.org/docs/SourceBasedCodeCoverage.html), if it is possible to map flow-generated code back to original code, we may not need to handle our own set of macro-based coverage tool which brings extra complicity.

Start with Clang's SourceBasedCodeCoverage documentation linked in the issue and inspect how FoundationDB's flow-generated code relates to its original source. Determine whether that mapping can replace the existing macro-based coverage tool, and document the feasibility and required scope.

Written by the indexing model from the issue text.