Investigate the need and impact of GC.Collect calls

[JanKrivanek]

Context

Reported by @davkean:

50% of builds show MSBuild nodes forcing a GC immediately after the build is done. At its worst case, this burned 3 cores for 3 seconds at a time that VS and the user think the build is done and VS is likely launching the app or user back to typing, so not really a good time to consume CPU.
In the few traces I looked at, these reduce memory by practically nothing, unnecessarily promotes, yet takes a long time. Those same sessions show they also have plenty of free physical memory. This GC.Collect should just be deleted and let the GC figure this out. I deleted all GC.Collect's from VS during Dev17 for the same reason.

Goal

Evaluate the impact of keeping versus removing the forced collections (we most likely should not need those, as either the next build call is quickly following, or we'll have the nodes eventually die of anyways).