TranslationUnitContext uses empty struct as a placeholder for forward declaration

[ForNeVeR]

See this snippet:

Cesium/Cesium.CodeGen/Contexts/TranslationUnitContext.cs

Lines 186 to 196 in 532fbec

	if (structType.Members.Count != 0 && structType.Identifier is not null)
	{
	if (_types.TryGetValue(structType.Identifier, out var existingType))
	{
	if (existingType is StructType existingStructType && existingStructType.Members.Count == 0)
	{
	existingStructType.Members = members;
	return existingType;
	}
	}
	}

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.