don't use type_parameter_def during astconv

astconv is called when converting the type-parameter, which leads to
a crash.

Fixes #26812.

src/test/compile-fail/issue-26812.rs

@@ -0,0 +1,12 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn avg<T=T::Item>(_: T) {} //~ ERROR associated type `Item` not found for `T`
+fn main() {}