Address PR feedback from Aleksey and add unit tests for both the tuple expression and parenting variable declaration so we test the operation tree for tuple conversions.

Fixes https://github.com/dotnet/roslyn/issues/19954 and https://github.com/dotnet/roslyn/issues/20125

Fixes https://github.com/dotnet/roslyn/issues/20312

See https://github.com/dotnet/roslyn/issues/18781#issuecomment-311428609 for the final design.
Fixes #18781

Fixes #19926

removed Operation cache. due to how we changed to hold onto child (Lazy(Of T)), we no longer need cache for children for a bound node

and that removes the need to have the old cache

This reverts commit 97deaeed.The fix is moved to a separate PR.

1. Add more unit tests
2. Switch the if condition to check for Bound.Interpolation instead of literal

Fixes #10856

```
    public interface ITupleExpression : IOperation
    {
        /// <summary>
        /// Elements for tuple expression.
        /// </summary>
        ImmutableArray<IOperation> Elements { get; }
    }
```

Fixes #18300