incr comp: rustc_clean/dirty auto assert

This adds auto-assertion to `rustc_clean/dirty` and also implements
more comprehensive testing for

 - src/test/incremental/hashes/enum_constructors.rs
 - src/test/incremental/hashes/enum_defs.rs
 - src/test/incremental/hashes/extern_mods.rs
 - src/test/incremental/hashes/inherent_impls.rs
 - src/test/incremental/hashes/statics.rs
 - src/test/incremental/hashes/struct_constructors.rs
 - src/test/incremental/hashes/type_defs.rs

trait_defs.rs and trait_impl.rs are blocked on a hard to triage
compiler ICE (at least hard for a newbie like me) having to do
with some DepNodes not getting computed for traits.
A FIXME has been added in the source to reflect this continued
work.

Author: vitiral

src/librustc/dep_graph/mod.rs

@@ -19,7 +19,7 @@ mod safe;
 mod serialized;
 
 pub use self::dep_tracking_map::{DepTrackingMap, DepTrackingMapConfig};
-pub use self::dep_node::{DepNode, DepKind, DepConstructor, WorkProductId};
+pub use self::dep_node::{DepNode, DepKind, DepConstructor, WorkProductId, label_strs};
 pub use self::graph::{DepGraph, WorkProduct, DepNodeIndex, DepNodeColor};
 pub use self::prev::PreviousDepGraph;
 pub use self::query::DepGraphQuery;