Add ability to ignore particular build warnings

[Daniel15]

Moved from dotnet/aspnetcore#900

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In csproj files, you can ignore particular build warnings with the <NoWarn> element (labelled as "Suppress warnings" in Visual Studio). For example, to ignore CS7035, you can either add <NoWarn>7035</NoWarn> to the csproj or pass it to MSBuild via /p:NoWarn=7035. This does not appear to be possible with project.json

cc @Eilon @dougbu

[smbecker]

Duplicate of #2042?

[davidfowl]

Yes it is!