incr.comp.: Cache the specialization_graph query.

[michaelwoerister]

Computing the specialization graph for a trait involves looking at lots of things. The result of the query should be easily cacheable:

rust/src/librustc/traits/specialize/specialization_graph.rs

Lines 39 to 66 in e65547d

	pub struct Graph {
	// all impls have a parent; the "root" impls have as their parent the def_id
	// of the trait
	parent: DefIdMap<DefId>,
	// the "root" impls are found by looking up the trait's def_id.
	children: DefIdMap<Children>,
	}
	/// Children of a given impl, grouped into blanket/non-blanket varieties as is
	/// done in `TraitDef`.
	struct Children {
	// Impls of a trait (or specializations of a given impl). To allow for
	// quicker lookup, the impls are indexed by a simplified version of their
	// `Self` type: impls with a simplifiable `Self` are stored in
	// `nonblanket_impls` keyed by it, while all other impls are stored in
	// `blanket_impls`.
	//
	// A similar division is used within `TraitDef`, but the lists there collect
	// together *all* the impls for a trait, and are populated prior to building
	// the specialization graph.
	/// Impls of the trait.
	nonblanket_impls: FxHashMap<fast_reject::SimplifiedType, Vec<DefId>>,
	/// Blanket impls associated with the trait.
	blanket_impls: Vec<DefId>,
	}

[wesleywiser]

I'll take this.