Generate diagnostic for explicitly AnyCPU NETFramework app with native dependencies

[nguerrera]

If you explicitly say PlatformTarget=AnyCPU but take dependencies on native assets from netframework app, nothing will be resolved as native-copy-local and the app is likely to fail. We should at least warn in this case:

Complication: We have to pass RIDs to restore to reason about native assets from project.assets.json. AnyCPU would imply that we're concerned if there are native assets for any RID, but that would bloat project.assets.json and slow down restore.

[natemcmaster]

As a part of this, I hope we can provide a clear story for library developers on how to make native packages work for AnyCPU.

For some data on this: I'm aware of at least 3 packages that address the AnyCPU situation already, and I've seen two different approaches.

Microsoft.Data.Sqlite and SQLitePCL.raw packages include MSBuild targets to copy native assets into $(OutputPath)/x86/sqlite3.dll and $(OutputPath)/x64/sqlite3.dll. At runtime, it detects the architecture and loads the native library from the right folder.

Microsoft.DiaSymReader.Native includes three binaries: Microsoft.DiaSymReader.Native.{x86, x64, arm}.dll At runtime, it detects the architecture and varies the p/invoked library name.