Refactor how impl self types are stored

In order to avoid a confusing use of the tcache, I added an extra
node ID field to trait refs. Now trait refs have a "ref ID" (the one
that resolve3 resolves) and an "impl ID" (the one that you look up
in the tcache to get the self type).

Closes #2434

src/libsyntax/parse/parser.rs

@@ -2189,7 +2189,7 @@ class parser {
             if option::is_none(ident) {
                 ident = some(vec::last(path.idents));
             }
-            some(@{path: path, id: self.get_id()})
+            some(@{path: path, ref_id: self.get_id(), impl_id: self.get_id()})
         } else { none };
         let ident = alt ident {
           some(name) { name }
@@ -2223,7 +2223,7 @@ class parser {
 
     fn parse_trait_ref() -> @trait_ref {
         @{path: self.parse_path_with_tps(false),
-          id: self.get_id()}
+          ref_id: self.get_id(), impl_id: self.get_id()}
     }
 
     fn parse_trait_ref_list() -> ~[@trait_ref] {