Fix handling of extension methods

ICSharpCode.Decompiler.Tests/TypeSystem/TypeSystemLoaderTests.cs

@@ -1316,6 +1316,19 @@ namespace ICSharpCode.Decompiler.Tests.TypeSystem
 			Assert.IsTrue(type.IsStatic);
 		}
 
+		[Test]
+		public void ExtensionMethodTest()
+		{
+			ITypeDefinition type = GetTypeDefinition(typeof(StaticClass));
+			var method = type.Methods.Single(m => m.Name == "Extension");
+
+			Assert.IsTrue(method.IsStatic);
+			Assert.IsTrue(method.IsExtensionMethod);
+			Assert.IsNull(method.ReducedFrom);
+
+			Assert.IsTrue(type.HasExtensionMethods);
+		}
+
 		[Test]
 		public void NoDefaultConstructorOnStaticClassTest()
 		{

ICSharpCode.Decompiler.Tests/TypeSystem/TypeSystemTestCase.cs

@@ -360,7 +360,11 @@ namespace ICSharpCode.Decompiler.Tests.TypeSystem
 		new event EventHandler Evt;
 	}
 
-	public static class StaticClass { }
+	public static class StaticClass
+	{
+		public static void Extension(this object inst) { }
+	}
+
 	public abstract class AbstractClass { }
 
 	public class IndexerNonDefaultName

ICSharpCode.Decompiler/TypeSystem/Implementation/MetadataMethod.cs

@@ -210,7 +210,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		}
 
 		IMember IMember.MemberDefinition => this;
-		IMethod IMethod.ReducedFrom => this;
+		IMethod IMethod.ReducedFrom => null;
 		TypeParameterSubstitution IMember.Substitution => TypeParameterSubstitution.Identity;
 
 		public ITypeDefinition DeclaringTypeDefinition {