C#: Fix type mention extraction of named types with nullability enabled

Author: tamasvajk

csharp/extractor/Semmle.Extraction.CSharp/SymbolExtensions.cs

@@ -497,13 +497,13 @@ public static IEnumerable<IParameterSymbol> GetParameters(this ISymbol parameter
         /// <summary>
         /// Holds if this symbol is a source declaration.
         /// </summary>
-        public static bool IsSourceDeclaration(this ISymbol symbol) => SymbolEqualityComparer.Default.Equals(symbol, symbol.OriginalDefinition);
+        public static bool IsSourceDeclaration(this ISymbol symbol) => SymbolEqualityComparer.IncludeNullability.Equals(symbol, symbol.OriginalDefinition);
 
         /// <summary>
         /// Holds if this method is a source declaration.
         /// </summary>
         public static bool IsSourceDeclaration(this IMethodSymbol method) =>
-            IsSourceDeclaration((ISymbol)method) && SymbolEqualityComparer.Default.Equals(method, method.ConstructedFrom) && method.ReducedFrom == null;
+            IsSourceDeclaration((ISymbol)method) && SymbolEqualityComparer.IncludeNullability.Equals(method, method.ConstructedFrom) && method.ReducedFrom == null;
 
         /// <summary>
         /// Holds if this parameter is a source declaration.

csharp/ql/test/library-tests/typeMentions/nullable-disable/TypeMentions.expected

@@ -1,6 +1,4 @@
 | Program.cs:6:24:6:27 | Base |
-| Program.cs:6:24:6:27 | Base |
-| Program.cs:8:23:8:24 | I0 |
 | Program.cs:8:23:8:24 | I0 |
 | Program.cs:9:21:9:22 | I1 |
 | Program.cs:13:12:13:15 | Void |

csharp/ql/test/library-tests/typeMentions/nullable-enable/TypeMentions.expected

@@ -1,8 +1,4 @@
 | Program.cs:6:24:6:27 | Base |
-| Program.cs:6:24:6:27 | Base |
-| Program.cs:6:24:6:27 | Base |
-| Program.cs:8:23:8:24 | I0 |
-| Program.cs:8:23:8:24 | I0 |
 | Program.cs:8:23:8:24 | I0 |
 | Program.cs:9:21:9:22 | I1 |
 | Program.cs:13:12:13:15 | Void |