Update S3949 and CFG creation for C# 9 syntax

[andrei-epure-sonarsource]

S3949 is generating MLIR instructions for all C# syntaxes, based on the CFG. When encountering an unknown syntax, it will throw an exception together with an AD0001 warning. It fails for C# 9 syntax.

error AD0001: Analyzer 'SonarAnalyzer.Rules.CSharp.CbdeHandlerRule' threw an exception of type 'SonarAnalyzer.CBDE.CbdeException' with message 'Top level error in CBDE handling: WithExpression 
 Details:  
 Inner exception:  
 Stack trace:    at SonarAnalyzer.ControlFlowGraph.CSharp.CSharpControlFlowGraphBuilder.BuildExpression(ExpressionSyntax expression, Block currentBlock) 
    at SonarAnalyzer.ControlFlowGraph.CSharp.CSharpControlFlowGraphBuilder.BuildVariableDeclarator(VariableDeclaratorSyntax variableDeclarator, Block currentBlock) 
    at SonarAnalyzer.ControlFlowGraph.CSharp.CSharpControlFlowGraphBuilder.BuildVariableDeclaration(VariableDeclarationSyntax declaration, Block currentBlock) 
    at SonarAnalyzer.ControlFlowGraph.CSharp.CSharpControlFlowGraphBuilder.BuildStatement(StatementSyntax statement, Block currentBlock) 
    at SonarAnalyzer.ControlFlowGraph.CSharp.CSharpControlFlowGraphBuilder.BuildStatements(IEnumerable`1 statements, Block currentBlock) 
    at SonarAnalyzer.ControlFlowGraph.CSharp.CSharpControlFlowGraphBuilder.BuildBlock(BlockSyntax block, Block currentBlock) 
    at SonarAnalyzer.ControlFlowGraph.CSharp.CSharpControlFlowGraphBuilder.BuildStatement(StatementSyntax statement, Block currentBlock) 
    at SonarAnalyzer.ControlFlowGraph.CSharp.CSharpControlFlowGraphBuilder.Build(SyntaxNode node, Block currentBlock) 
    at SonarAnalyzer.ControlFlowGraph.AbstractControlFlowGraphBuilder.Build() 
    at SonarAnalyzer.ControlFlowGraph.CSharp.CSharpControlFlowGraph.Create(CSharpSyntaxNode node, SemanticModel semanticModel) 
    at SonarAnalyzer.CBDE.MlirExporter.ExportFunction(MethodDeclarationSyntax method) 
    at SonarAnalyzer.CBDE.CbdeHandler.ExportFunctionMlir(SyntaxTree tree, SemanticModel model, MlirExporterMetrics exporterMetrics, String mlirFileName) 
    at SonarAnalyzer.CBDE.CbdeHandler.<RegisterMlirAndCbdeInOneStep>b__31_0(CompilationAnalysisContext c)'. [C:\Workspace\sonar-dotnet\sonaranalyzer-dotnet\its\sources\Net5\Net5.csproj

Note: The regression ITs are considering AD0001 warnings as errors. Thus when running the Net5 integration test, we disable S3949. We should re-enable it after fixing this issue (see sonaranalyzer-dotnet/its/regression-test.ps1).