Add regression test for inlined doc comment on impl block

Author: GuillaumeGomez

src/test/rustdoc/auxiliary/reexport-doc-aux.rs

@@ -0,0 +1,5 @@
+pub struct Foo;
+
+impl Foo {
+    pub fn foo() {}
+}

src/test/rustdoc/reexport-doc.rs

@@ -0,0 +1,8 @@
+// aux-build:reexport-doc-aux.rs
+
+extern crate reexport_doc_aux as dep;
+
+// @has 'reexport_doc/struct.Foo.html'
+// @count - '//p' 'These are the docs for Foo.' 1
+/// These are the docs for Foo.
+pub use dep::Foo;