Implement CheckInvariant for C# AST

[siegfriedpammer]

While debugging #1628 I came across the situation that the assumption we use in some C# AST transforms (especially relevant for DeclareVariables), "that AstNode.FirstChild is always the first sub-expression/node that will be evaluated when the program is executed" is no longer true.

For example: BinaryOperatorExpression.Left == BinaryOperatorExpression.FirstChild is no longer true for some nodes in the tree in the final output visitor run.

This causes the problem that VariableToDeclare.FirstUse points to an expression in the middle of the operators and not to the first use.

Note that the DeclareVariables.Analyze run we do in PatternStatementTransform did not observe this problem.

We need some mechanism similar to the ILAst CheckInvariant structure to verify our assumptions about the structure of the C# AST.

[siegfriedpammer]

#2033: This was caused because we generally assume that all C# Statement nodes that represent loops have their loop BlockContainer annotated. In this case the annotation was missing. An assertion could easily find such bugs.