Add AnalysisLevel MSBuild property

[jmarolf]

There will be a central MSBuild property that can be used to configure what /warnversion is passed to the compiler.
Tentatively call AnalysisLevel for now

• This will need to be plumbed through VS tooling including property pages
• Would also like to remove “Warning Level” from the VS Build property pages in favor of this

Related to dotnet/roslyn#45701

[sbomer]

We plan to respect AnalysisLevel for ILLink as well. Would you be able to share more details on the VS UI for this? I'm wondering if it's going to set AnalysisLevel, and whether the plan is still to remove "Warning Level" in favor of a new UI, now that the compiler implementation is reusing the /warn option instead of introducing a new /warnversion.

[jmarolf]

@sbomer as of now, the design is that /warn is only for controlling compiler produced diagnostics. AnalysisLevel is for controlling third-party tools that feed in diagnostics like analyzers (and presumably ILLink). /warn will still exist for compiler config (behind the advanced button in the UI) but I would expect the new AnalysisLevel drop down to be the main way users would configure this.