Need mechanism to disable trim warnings from the linker, while keeping them from the analyzer

Some platforms (e.g., blazor) may want to provide trim warnings for user code, but not for referenced libraries. It would be useful if they could just rely on the analyzer.

Since this is likely a rarely used feature, I don't think it needs that much design work. I propose that we add a warn level to ILLink, and have `0` mean suppress all trim warnings. A corresponding MSBuild property, ILLinkWarnLevel, would control this. Then, users could disable the ILLink warnings without affected the analyzer.