Add analyzer to flag converting from DllImport to GeneratedDllImport

[elinor-fung]

This does a coarse check to flag DllImport methods where switching to GeneratedDllImport can be useful. Flags the DllImport method if it has one of:

• SetLastError=true
• PreserveSig=false
• by-ref parameters
• parameter or return type that is not considered blittable

Disabled by default. To enable, add to editorconfig:
dotnet_diagnostic.DLLIMPORTGENANALYZER015.severity = <severity>

Was able to run against a local build of libraries (not just NetCoreApp) on Windows (using https://github.com/elinor-fung/runtime/tree/useDllImportGenerator)

• flagged 1564 methods with warnings
• -os Linux : 1460 warnings
• -os OSX: 1210 warnings

cc @AaronRobinsonMSFT @jkoritzinsky

[jkoritzinsky]

Do we want to add a code fix that does the conversion as well?

[elinor-fung]

I think we want a fixer eventually. For our current target of the libraries, since they are used for many target frameworks and we only support net5.0+, we'd have to use the right target framework preprocessor defines (or add a define specifically for when the P/Invoke generator is enabled). I didn't add a fixer here, since I honestly didn't want to deal with that yet and I think just flagging them provides some value right now.