ApiCompat loading of Roslyn on NETFramework is unreliable

[ericstj]

APICompat will load Roslyn once since it binds to it directly and doesn't use AppDomains to isolate different bindings.

This can lead to torn state if we happen to only bind to one Roslyn assembly and later bind to a different one. See #45872 (comment)

To fix this in APICompat we can do one of two things:

1. Ensure we bind to a single version of all of roslyn's assemblies by either forcing a load, or using a write-once static to capture the Roslyn assemblies path.
2. Additionally to 1, create an appdomain to host APICompat assemblies if the version of roslyn specified differs from the one already loaded.

[dotnet-policy-service]

@dotnet/area-infrastructure-libraries a new issue has been filed in the ApiCompat area, please triage