fix InvalidCastException in GeneratePartialClassStub: not every IMethod corresponds to a MethodDeclaration (there are ConstructorDeclaration and DestructorDeclaration, for ex.)

Author: siegfriedpammer

src/AddIns/BackendBindings/CSharpBinding/Project/Src/CSharpLanguageBinding.cs

@@ -74,7 +74,7 @@ static string GeneratePartialClassContextStub(ICodeContext context)
 				return "";
 			var builder = new TypeSystemAstBuilder();
 			MethodDeclaration decl;
-			if (member is IMethod) {
+			if (member.SymbolKind == SymbolKind.Method) {
 				// If it's a method, convert it directly (including parameters + type parameters)
 				decl = (MethodDeclaration)builder.ConvertEntity(member);
 			} else {