Validate that Roslyn and MSBuild and VS have the same System.Collections.Immutable version

[AB#1245899](https://devdiv.visualstudio.com/0bdbc590-a062-4c3f-b0f6-9383f67865ee/_workitems/edit/1245899) was a customer-visible regression that occurred because MSBuild used an older copy of `System.Collections.Immutable` than Roslyn, causing build-time failures in a task that referenced Roslyn.

The policy has been that Roslyn owns the Immutable version and everything should match it, but that was missed this time.

dotnet/roslyn#49336 adds a comment but that's not validation.

We should have validation that runs in an automated way that checks these versions (assembly version is probably sufficient; we can likely differ in prereleases most of the time as long as we all converge to runtime GA at VS GA).

It's not totally clear to @JoeRobich or me where this should go, so the first order of business is figuring that out.

No file, test, or entry point is identified; first determine where MSBuild, Roslyn, and Visual Studio version validation belongs and inspect the existing versioning and build validation setup. Done means an automated check verifies that their System.Collections.Immutable assembly versions converge as required and catches mismatches.

Written by the indexing model from the issue text.