Infra: Reduce build time to perform CLR up-to-date check

The time being measured is the time needed to build the clr subset, after the subset has been completely built already.

If this time were low enough, under 5 seconds or so, it would be feasible to add it as a dependency to other subsets. As it is, it takes roughly 51s on my machine.

Ongoing list of work items, to be updated until we're at 5 secons or less:

• Incrementally crossgen System.Private.CoreLib
• clretwrc appears to always be rebuilt
• Switch to Ninja in Windows by default
  • This will require a workflow for building and preserving VS project files
• Consider changing the default meaning of clr partition
  • Linux DAC is built by default on Windows (this is expensive. We could try to make it cheaper, or move it out of dev build)
  • CLR managed tools are built as part of CLR