Fix for middle::reachable + better comments and tests

In `middle::reachable` mark default impl of a trait method as reachable if this trait method is used from inlinable code

src/test/auxiliary/issue-11225-3.rs

@@ -0,0 +1,29 @@
+// 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.
+
+trait PrivateTrait {
+    fn private_trait_method(&self);
+}
+
+struct PrivateStruct;
+
+impl PrivateStruct {
+    fn private_inherent_method(&self) { }
+}
+
+impl PrivateTrait for PrivateStruct {
+    fn private_trait_method(&self) { }
+}
+
+#[inline]
+pub fn public_generic_function() {
+    PrivateStruct.private_trait_method();
+    PrivateStruct.private_inherent_method();
+}

src/test/run-pass/issue-11225-3.rs

@@ -0,0 +1,19 @@
+// 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.
+
+// aux-build:issue-11225-3.rs
+
+// pretty-expanded FIXME #23616
+
+extern crate issue_11225_3;
+
+pub fn main() {
+    issue_11225_3::public_generic_function();
+}