CSharpConvertIfToSwitchCodeRefactoringProvider does not work with top-level statements

[AlekseyTs]

Start with the following compilation unit:

var e = new ET1();

if (e == ET1.A)
{
}
else if (e == ET1.C)
{
}

enum ET1
{
    A,
    B,
    C,
}

Try using "If to switch" refactoring.

Observed:

System.InvalidCastException : Unable to cast object of type 'Microsoft.CodeAnalysis.CSharp.Syntax.SwitchStatementSyntax' to type 'Microsoft.CodeAnalysis.CSharp.Syntax.MemberDeclarationSyntax'.
   at Microsoft.CodeAnalysis.CSharp.CSharpSyntaxRewriter.VisitListElement[TNode](TNode node)
   at Microsoft.CodeAnalysis.CSharp.CSharpSyntaxRewriter.VisitList[TNode](SyntaxList`1 list)
   at Microsoft.CodeAnalysis.CSharp.CSharpSyntaxRewriter.VisitCompilationUnit(CompilationUnitSyntax node)
   at Microsoft.CodeAnalysis.CSharp.Syntax.CompilationUnitSyntax.Accept[TResult](CSharpSyntaxVisitor`1 visitor)
   at Microsoft.CodeAnalysis.CSharp.CSharpSyntaxRewriter.Visit(SyntaxNode node)
   at Microsoft.CodeAnalysis.CSharp.Syntax.SyntaxReplacer.Replacer`1.Visit(SyntaxNode node)
   at Microsoft.CodeAnalysis.CSharp.Syntax.SyntaxReplacer.Replace[TNode](SyntaxNode root,IEnumerable`1 nodes,Func`3 computeReplacementNode,IEnumerable`1 tokens,Func`3 computeReplacementToken,IEnumerable`1 trivia,Func`3 computeReplacementTrivia)
   at Microsoft.CodeAnalysis.CSharp.CSharpSyntaxNode.ReplaceCore[TNode](IEnumerable`1 nodes,Func`3 computeReplacementNode,IEnumerable`1 tokens,Func`3 computeReplacementToken,IEnumerable`1 trivia,Func`3 computeReplacementTrivia)
   at Microsoft.CodeAnalysis.SyntaxNodeExtensions.ReplaceNode[TRoot](TRoot root,SyntaxNode oldNode,SyntaxNode newNode)
   at async Microsoft.CodeAnalysis.ConvertIfToSwitch.AbstractConvertIfToSwitchCodeRefactoringProvider`4.UpdateDocumentAsync[TIfStatementSyntax,TExpressionSyntax,TIsExpressionSyntax,TPatternSyntax](<Unknown Parameters>)
   at async Microsoft.CodeAnalysis.CodeActions.CodeAction.GetChangedSolutionAsync(<Unknown Parameters>)
   at async Microsoft.CodeAnalysis.CodeActions.CodeAction.ComputeOperationsAsync(<Unknown Parameters>)
   at async Microsoft.CodeAnalysis.CodeActions.CodeAction.GetPreviewOperationsAsync(<Unknown Parameters>)
   at async Microsoft.CodeAnalysis.Editor.Implementation.Suggestions.SuggestedAction.GetPreviewResultAsync(<Unknown Parameters>)
   at async Microsoft.CodeAnalysis.Editor.Implementation.Suggestions.SuggestedActionWithNestedFlavors.<>c__DisplayClass11_0.<GetPreviewAsync>b__0(<Unknown Parameters>)
   at async Microsoft.CodeAnalysis.Extensions.IExtensionManagerExtensions.PerformFunctionAsync[T](<Unknown Parameters>)
   at Microsoft.VisualStudio.Telemetry.WindowsErrorReporting.WatsonReport.GetClrWatsonExceptionInfo(Exception exceptionObject)

Relates to #43563 (test plan for "top-level statements")

[AlekseyTs]

CC @sharwell