Associate each path in a use declaration with the item in the AST map

Currently, for `use` declarations with multiple paths, only the `use`
item itself is saved in the AST map, not the individual path nodes. This
can lead to a problem when a span of a specific path node is needed.

For example, #24818 caused an ICE because of this, in
`ImportResolver::check_for_conflicting_import()`.

Fixes #25763.

src/test/compile-fail/use-paths-as-items.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.
+
+// Each path node in a `use` declaration must be treated as an item. If not, the following code
+// will trigger an ICE.
+//
+// Related issue: #25763
+
+use std::{mem, ptr};
+use std::mem; //~ ERROR has already been imported
+
+fn main() {}