Add source coverage output to coverage task

[ranweiler]

Our coverage task emits module+offset binary coverage, and leverages existing Sancov instrumentation when available.

For many SDL tasks, source file+line coverage will be most actionable and informative. Keep binary coverage reporting, but also report source coverage, when available.

We currently have library code that can emit source coverage for PE targets with PDBs, but it is not yet integrated into the coverage task. To fully support source coverage, we need the following:

• Define a source-based coverage format #1381
• Integrate source-based coverage reporting for Windows #1378
• Implement Linux source-based coverage #1379
• Integrate source-based coverage reporting for Linux #1380

[ranweiler]

This work is complete! See linked issues for details.