`TranslationUnitContext` uses empty struct as a placeholder for forward declaration

[ForNeVeR]

See this snippet: https://github.com/ForNeVeR/Cesium/blob/532fbec3dcae1d5084971ab440f0ae865146d199/Cesium.CodeGen/Contexts/TranslationUnitContext.cs#L186-L196

From this, it looks like we use an empty struct (with no members) as a placeholder for a forward declaration, or something similar.

Empty structs is a useful standard extension, and we may choose to implement it later; thus this will break.

We should improve the architecture here and make the placeholder more explicit.