rustdoc: do not strip blanket impls in crate of origin

In `impl<T> Trait for T`, the blanket type parameters `T` were
recognized as "local" and "not exported", so these impls were
thrown out.

Now we check if they are generic, and keep them in that case.

Fixes: #29503

src/test/rustdoc/issue-29503.rs

@@ -0,0 +1,26 @@
+// Copyright 2016 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.
+
+use std::fmt;
+
+// @has issue_29503/trait.MyTrait.html
+pub trait MyTrait {
+    fn my_string(&self) -> String;
+}
+
+// @has - "//ul[@id='implementors-list']/li" "impl<T> MyTrait for T where T: Debug"
+impl<T> MyTrait for T where T: fmt::Debug {
+    fn my_string(&self) -> String {
+        format!("{:?}", self)
+    }
+}
+
+pub fn main() {
+}