Use HashSet instead of List for _virtual_methods (dotnet/linker#2976)

* Use a HashSet instead of List for MarkStep._virtual_methods

Commit migrated from dotnet/linker@6252a21

src/tools/illink/src/linker/Linker.Steps/MarkStep.cs

@@ -59,7 +59,7 @@ protected LinkContext Context {
 		}
 
 		protected Queue<(MethodDefinition, DependencyInfo, MessageOrigin)> _methods;
-		protected List<(MethodDefinition, MarkScopeStack.Scope)> _virtual_methods;
+		protected HashSet<(MethodDefinition, MarkScopeStack.Scope)> _virtual_methods;
 		protected Queue<AttributeProviderPair> _assemblyLevelAttributes;
 		readonly List<AttributeProviderPair> _ivt_attributes;
 		protected Queue<(AttributeProviderPair, DependencyInfo, MarkScopeStack.Scope)> _lateMarkedAttributes;
@@ -224,7 +224,7 @@ internal DynamicallyAccessedMembersTypeHierarchy DynamicallyAccessedMembersTypeH
 		public MarkStep ()
 		{
 			_methods = new Queue<(MethodDefinition, DependencyInfo, MessageOrigin)> ();
-			_virtual_methods = new List<(MethodDefinition, MarkScopeStack.Scope)> ();
+			_virtual_methods = new HashSet<(MethodDefinition, MarkScopeStack.Scope)> ();
 			_assemblyLevelAttributes = new Queue<AttributeProviderPair> ();
 			_ivt_attributes = new List<AttributeProviderPair> ();
 			_lateMarkedAttributes = new Queue<(AttributeProviderPair, DependencyInfo, MarkScopeStack.Scope)> ();
@@ -3352,7 +3352,6 @@ void MarkBaseMethods (MethodDefinition method)
 				if (base_method.DeclaringType.IsInterface && !method.DeclaringType.IsInterface) {
 					// These are all virtual, no need to check IsVirtual before adding to list
 					_virtual_methods.Add ((base_method, ScopeStack.CurrentScope));
-					// _virtual_methods is a list and might have duplicates, but it's mostly just used for override validation, so it shouldn't matter
 					continue;
 				}