fsc.exe - Parallel Type-Checking of implementation files if signature files exist

[TIHan]

Building on the ideas of #10201 - there might be a way we can do parallel type-checking for implementation files that have backing signature files. This requires a bit more research to see if this is possible, but in theory it should be.

The compiler would do a pass at type-checking all files minus the implementation files that have a corresponding signature file. After that pass completes, then we type-check those implementation files and continue.

[cartermp]

#11152 implements this

[T-Gro]

This has been done.