Follow up work items for extension operators

[AlekseyTs]

The issue is created to track a follow up for the following PROTOTYPE comments:

Binder_Operators.cs(168):// PROTOTYPE: If this result is bad, we might want to stick with bad non-extension result as we do for static extensions below.
Binder_Operators.cs(172):// PROTOTYPE: Add test coverage for the choice between two bad results
Binder_Operators.cs(3123):// PROTOTYPE: If this result is bad, we might want to stick with bad non-extension result as we do for static extensions below.
Binder_Operators.cs(3127):// PROTOTYPE: Add test coverage for the choice between two bad results
Binder_Operators.cs(3651):continue; // PROTOTYPE: Test this code path for actually interesting scenarios around RefKind which have an observable effect vs. just an optimization.
Binder_Operators.cs(3663):continue; // PROTOTYPE: Add test coverage for this code path

The comments in source code now reference this issue.