Generate InAttribute for .NET Standard 1.0 #97

Reproduction Steps

Can not compile the next code for .NET Standard 1.0 without InAttribute.

public static class Foo
{
    public static ref readonly object GetPinnableReference(this object bar) => throw new();
}

Expected Behavior

Successful compilation.

Actual Behavior

Next compiler error

Error CS0518 Predefined type 'System.Runtime.InteropServices.InAttribute' is not defined or imported (netstandard1.0)

System info

• PolySharp NuGet version LAST STABLE
• Operating system version Windows 10
• Visual Studio version LAST STABLE