Turn netcore vs netfx apicompat checks into errors

[weshaggard]

To help with compat issues like https://github.com/dotnet/corefx/pull/21388/files#r123627397 we need to turn the ApiCompat runs on all the time and make them errors.

The start of this work is in branch https://github.com/weshaggard/corefx/tree/EnableNetFxApiCompat but it still needs some work before it passes.

cc @ericstj @danmosemsft

[weshaggard]

This is to make any additions to shims\ApiCompatBaseline.netcoreapp.netfx461.txt cause a build error. Treat it as a baseline.

Then analyze what is in that baseline and fix (or log issues) as needed so it's an acceptable baseline,.

We don't see this as essential for 2.1.

[ViktorHofer]

I don't think we absolutely need this. Should we close it? @danmosemsft @ericstj

[danmoseley]

Your call