Refactor IR graph traversal infrastructure

This makes the IR traversal infrastructure generic. It makes it so we can use
the same traversal code for whitelisting traversals and asserting no dangling
item references. Therefore the queue of items to visit is generic (whitelisting
uses DFS, while asserting against dangling uses BFS), the storage for the seen
set (whitelisting uses a simple set, asserting against dangling uses a map from
item to the item from which it was discovered).

It also introduces the concept of different kinds of edges in the IR graph, and
the ability to choose which edges to follow. This means we can simplify
non-transitive whitelisting to a simple function that always returns "no do not
follow this edge". It plays an important part for future analysis of which
template declaration type parameters are used or not.

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