* Change the IOperation unit test dump format for child nodes

* fixing the newly merged tests.

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 #19927

Make ILambdaExpression.Type and IUnboundLambdaExpression.Type return null, to be consistent with the SemanticModel.

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 #19790

Fixes #18300

The current IOperation API implementation of BoundDelegateCreationExpression has a bunch of issues and needs to be redesigned. This is tracked by #8897 and will be addressed post 15.3. Meanwhile, to unblock analyzers on code containing delegate creation expressions with lambda arguments, this bound node has been switched to OperationKind.None with override for Children property.

Fixes the first repro case provided in #8884

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

Add arglist test and revert changes to BoundByRefArgumentWithCopyBack as it is never exposed in the operation tree

Ensure that we get a well formed child IOperation tree even for bound nodes for whom IOperation support is not yet implemented

This ensures that the analyzer driver/operation walker is able to find all the descendant IOperation nodes within these not yet implemented features. We should remove the IOperationWithChildren interface once we have designed all the IOperation APIs.

TODO: Implement the VB part.

Fixes #8884