Missing `unmanaged` Constraint When Projecting Types for Pointers

[JasonBock]

I think this is a case where this may be a warning, and it only shows up during the code gen test run. The ProjectedTypesGeneratorTests.CreateWithPointersAndGenericsAndUnmanagedConstraintAsync() test passes, but it seems like the generated code for the delegate and Argument class should have where T : unmanaged on it.

[JasonBock]

I think by doing this feature, that will automatically take care of this.

[JasonBock]

Since I won't do #244, looks like I'll have to make this small fix.