Measure the impact of multithreading with fixed tasks.

[AR-May]

The prototype https://github.com/dotnet/msbuild/tree/dev/AR-May/multi-threaded-msbuild-3 includes many task fixes that enable it to run real-world projects. These updated tasks can be used to create PerfStar and exp insertion runs, allowing us to quickly measure the impact of multithreading. However, the engine changes from the prototype should not be reused. We should instead rely on existing work that is checked in MSBuild. This will help us generate early estimates of multithreading performance impact.

[AR-May]

Enlightened many msbuild tasks and measured build performance with them.
Reporting numbers here.

Next steps:

• Collect more detailed data. Measure how much time is spent in non-task-execution phases of the build
• Measure full framework project
• Enlighten common sdk tasks and measure.