When targeting a .NET 4.0 or 4.5 project it's not possible by default to use the Interpolated Strings and their FormattableString and IFormattable counterparts. By default*.***
But, this is only because these two types didn't ship with the older version of the .NET Framework. If you make sure these classes are available to the C#6 compiler, it will happily produce an assembly which is compatible with the older versions of the .NET framework, since these interpolated strings are a language feature and not dependent on the Common Language Runtime.
You'll need these classes to pass a CultureInfo in order to satisfy Code Analysis 1305.
Someone beat me to creating a NuGet Package of the classes required, which makes this as simple as running: