C#: Fix nullable warnings and some code quality issues

Author: tamasvajk

csharp/extractor/Semmle.Extraction.CIL/Entities/ConstructedType.cs

@@ -1,5 +1,4 @@
 using System;
-using Microsoft.CodeAnalysis;
 using System.Linq;
 using System.Collections.Generic;
 using System.IO;

csharp/extractor/Semmle.Extraction.CIL/Entities/Instruction.cs

@@ -481,7 +481,7 @@ public IEnumerable<IExtractionProduct> JumpContents(Dictionary<int, Instruction>
                 // TODO: Find a solution to this.
 
                 // For now, just log the error
-                Cx.Cx.ExtractionError("A CIL instruction jumps outside the current method", "", Extraction.Entities.GeneratedLocation.Create(Cx.Cx), "", Util.Logging.Severity.Warning);
+                Cx.Cx.ExtractionError("A CIL instruction jumps outside the current method", null, Extraction.Entities.GeneratedLocation.Create(Cx.Cx), "", Util.Logging.Severity.Warning);
             }
         }
     }

csharp/extractor/Semmle.Extraction.CIL/Entities/NamedTypeIdWriter.cs

@@ -1,4 +1,3 @@
-using Microsoft.CodeAnalysis;
 using System.Linq;
 using System.IO;
 

csharp/extractor/Semmle.Extraction.CIL/Entities/NoMetadataHandleType.FullyQualifiedNameParser.cs

@@ -22,7 +22,7 @@ internal sealed partial class NoMetadataHandleType
         private class FullyQualifiedNameParser
         {
             public string ShortName { get; internal set; }
-            public string? AssemblyName { get; internal set; }
+            public string? AssemblyName { get; private set; }
             public IEnumerable<string>? TypeArguments { get; internal set; }
             public string? UnboundGenericTypeName { get; internal set; }
             public string ContainerName { get; internal set; }

csharp/extractor/Semmle.Extraction.CIL/Entities/NoMetadataHandleType.cs

@@ -66,7 +66,7 @@ public NoMetadataHandleType(Context cx, string originalName) : base(cx)
 
         private void Populate()
         {
-            if (isContainerNamespace)
+            if (ContainingNamespace is object)
             {
                 Cx.Populate(ContainingNamespace);
             }

csharp/extractor/Semmle.Extraction.CIL/Entities/SignatureDecoder.cs

@@ -1,7 +1,6 @@
 using System.Reflection.Metadata;
 using System.Collections.Immutable;
 using System.IO;
-using System.Linq;
 
 namespace Semmle.Extraction.CIL.Entities
 {

csharp/extractor/Semmle.Extraction.CIL/Entities/TypeDefinitionType.cs

@@ -1,12 +1,10 @@
 using System;
-using Microsoft.CodeAnalysis;
-using System.Reflection.Metadata;
-using System.Collections.Immutable;
 using System.Linq;
 using System.Collections.Generic;
 using System.Reflection;
 using System.IO;
 using System.Reflection.Metadata.Ecma335;
+using System.Reflection.Metadata;
 
 namespace Semmle.Extraction.CIL.Entities
 {

csharp/extractor/Semmle.Extraction.CIL/Entities/TypeReferenceType.cs

@@ -1,7 +1,5 @@
 using System;
-using Microsoft.CodeAnalysis;
 using System.Reflection.Metadata;
-using System.Collections.Immutable;
 using System.Linq;
 using System.Collections.Generic;
 using System.IO;

csharp/extractor/Semmle.Extraction.CIL/PDB/MetadataPdbReader.cs

@@ -76,7 +76,7 @@ public MetadataPdbReader(MetadataReaderProvider provider)
                     out provider,
                     out _))
                 {
-                    return new MetadataPdbReader(provider);
+                    return new MetadataPdbReader(provider!);
                 }
             }
 

csharp/extractor/Semmle.Extraction.CIL/PDB/Method.cs

@@ -1,5 +1,4 @@
 using System.Collections.Generic;
-using System.Reflection.Metadata;
 using System.Linq;
 
 namespace Semmle.Extraction.PDB