rustdoc: External module item links to the module contents. Fixes #12926.

the basic strategy is to distinguish `mod foo;` from `mod foo {...}`
by checking if the span for the module item and module contents is
in different files. if it's the case, we prefer module contents.

it is technically possible to fix #12926 without changing the AST,
probably by checking the individual items' span. this is not without
a problem though, since it is possible that some items inside
`mod foo {...}` may have originated from other file (e.g. `include!`).
therefore it is better to record both spans explicitly.

src/librustdoc/doctree.rs

@@ -19,7 +19,8 @@ use syntax::ast::{Ident, NodeId};
 pub struct Module {
     pub name: Option<Ident>,
     pub attrs: Vec<ast::Attribute>,
-    pub where: Span,
+    pub where_outer: Span,
+    pub where_inner: Span,
     pub structs: Vec<Struct>,
     pub enums: Vec<Enum>,
     pub fns: Vec<Function>,
@@ -42,7 +43,8 @@ impl Module {
             name       : name,
             id: 0,
             vis: ast::Inherited,
-            where: syntax::codemap::DUMMY_SP,
+            where_outer: syntax::codemap::DUMMY_SP,
+            where_inner: syntax::codemap::DUMMY_SP,
             attrs      : Vec::new(),
             structs    : Vec::new(),
             enums      : Vec::new(),