Fix compound object field designator

[kant2002]

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.

Cesium/Cesium.CodeGen/Ir/Expressions/CompoundObjectFieldInitializer.cs

Line 14 in 398ed5e

internal Designation Designation;

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.