Fix compound object field designator

Currently the object field initializer uses designator from AST. That leads to polluted emitter in `CompoundObjectInitializationExpression` which known about what kind of initializer we have.

https://github.com/ForNeVeR/Cesium/blob/398ed5e3f73dc2cf551914eb07aa0ce6eeebe622/Cesium.CodeGen/Ir/Expressions/CompoundObjectFieldInitializer.cs#L14

Instead of that, I think proper way to handle that would be encode initialization at that AST->IR translation, or create special IR translation phase which desugar expression to primitive object field assigning.

Start with Cesium.CodeGen/Ir/Expressions/CompoundObjectFieldInitializer.cs at the referenced line, then trace the AST-to-IR translation and CompoundObjectInitializationExpression emitter. Determine how compound object field initialization should be encoded or desugared so the emitter no longer relies on the AST designator; done means the resulting IR represents primitive object-field assignments without polluted emitter output.

Written by the indexing model from the issue text.