Expand the ShimLayer with ITypeSymbol.NullableAnnotation

[Tim-Pohlmann]

ITypeSymbol.NullableAnnotation is currently not part of the ShimLayer. Adding it, will make checking for Nullables easier and more robust.
See also:
#3273 (comment)
#6753 (comment)

[martin-strecker-sonarsource]

Something like in TypeInfoExtensions needs to be created:

public static class ITypeSymbolExtensions
{
    public NullableAnnotation NullableAnnotation(this ITypeSymbol symbol) { }
}

It is a bit easier as TypeInfoExtensions.Nullability(this TypeInfo typeInfo) but still needs the enum to enum conversion. Maybe one of the LightupHelpers.CreateSyntaxPropertyAccessor already does what we need (there is an overload that takes a type and calls Convert).

[Corniel]

@Tim-Pohlmann I think we've solved this with #6952 . :)

[Tim-Pohlmann]

That's great news. Thank you!