IsInvalid -> HasDiagnostics
removed HasChildren interface and merged it to IOperation

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

FieldInitializerInCreation and PropertyInitializerInCreation are no longer used, so this change removes them.
Fixes https://github.com/dotnet/roslyn/issues/19300

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

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

Allow people to create SyntaxTokenLists at common layer, without having to go through VB or C# layers.

Fixes #19926

Fixes #19927

Remove IUnboundLambdaExpression from IOperation API, return an ILambdaExpression for UnboundLambda nodes.

Fixes #19902

Analysis of any code within a local function with IOperation analyzers will produce false or missing diagnostics, as the operation tree doesn't contain any operation nodes within the BoundLocalFunctionStatement. This can be a dogfood blocker for IOperation analyzers on real world code - we had few false reports due this on the Roslyn repo as we use this new language feature, and corresponding rules had to be disabled in the repo.

Fixes #10856

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

Fixes #18300

Re-enable roslyn analyzers on Roslyn.sln and update public api file for an IOperation API change merged yesterday (#19278).