CSharpImmutableObjectMethodAnalyzer does not work when MS.CA.Workspaces is not installed

[svick]

Analyzer package

Microsoft.CodeAnalysis.Analyzers

Analyzer

CSharpImmutableObjectMethodAnalyzer

Repro steps

1. Create a new C# project.

2. Install the package Microsoft.CodeAnalysis.CSharp, but not Microsoft.CodeAnalysis.CSharp.Workspaces

3. Write code that doesn't use the returned value from a method on an immutable object:

   var namespaceNode = SyntaxFactory.NamespaceDeclaration(
       SyntaxFactory.ParseName("Root.MyNamespace1"));
   var classNode = SyntaxFactory.ClassDeclaration("MyClass");
   
   namespaceNode.AddMembers(classNode);

Expected behavior

The last line reports:

RS1014 'NamespaceDeclarationSyntax' is immutable and 'AddMembers' will not have any effect on it. Consider using the return value from 'AddMembers'.

Actual behavior

No warning.

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This is caused by this line, which means that when symbols from MS.CA.Workspaces are not found, the analyzer does nothing.

[Evangelink]

Relates to #4564 and #615