Use Csc task to create CLI arguments for Roslyn

[amis92]

See #183 (comment)

[amis92]

The actual task code used would be ManagedCompiler.cs's ExecuteTool override:

https://github.com/dotnet/roslyn/blob/082d02d0616442b0370f28eecc981f8bcbec48d2/src/Compilers/Core/MSBuildTask/ManagedCompiler.cs#L459-L470

We should pass ProvideCommandLineArgs=true and SkipCompilerExecution=true. This will result in all Roslyn CLI arguments output into CommandLineArgs item.