There are couple of changes here:
1. API change: `ImmutableArray<ISymbolInitializer> MemberInitializers` is changed to `ImmutableArray<IOperation> Initializers`.
2. Implementation changes:
   1. Instead of returning the member initializers as synthesized ISymbolInitializer nodes, we now return member intializers as IAssignmentExpression nodes. This ensures completeness of IOperation tree.
   2. Now we also return the collection intializer expressions within an object creation expression.

Fixes #18115

There are 2 bugs still affecting this area:
1. #18781: IOperation API shape for collection initializer expressions
2. #19276: Missing Field/Property reference expression nodes in object creation initializer node