Fix the problem with check-fast randomly failing

Trans has been assuming that tag node id's are unique across crates and they
are not so, depending on which way the wind is blowing, it would choose to use
a crate-local tag variant instead of the correct one from std.

No test case since I can't come up with a reliable one that compiles in a
reasonable amount of time.

src/comp/syntax/ast_util.rs

@@ -184,6 +184,18 @@ fn eq_ty(&&a: @ty, &&b: @ty) -> bool { ret std::box::ptr_eq(a, b); }
 
 fn hash_ty(&&t: @ty) -> uint { ret t.span.lo << 16u + t.span.hi; }
 
+fn hash_def_id(&&id: def_id) -> uint {
+    id.crate as uint << 16u + (id.node as uint)
+}
+
+fn eq_def_id(&&a: def_id, &&b: def_id) -> bool {
+    a == b
+}
+
+fn new_def_id_hash<@T>() -> std::map::hashmap<def_id, T> {
+    std::map::mk_hashmap(hash_def_id, eq_def_id)
+}
+
 fn block_from_expr(e: @expr) -> blk {
     let blk_ = default_block([], option::some::<@expr>(e), e.id);
     ret {node: blk_, span: e.span};