Auto merge of #663 - fitzgen:how-many-stylo-bugs-to-screw-in-a-lightbulb, r=emilio

Fix blacklisting and fix instantiations in the template analysis

For correctness, I now realize that we need to consider the full transitive closure of whitelisted items, including blacklisted items that we would otherwise not consider as whitelisted. We need this so that the data flow propagates properly through dependencies that may or may not be blacklisted.

This means that tracing infrastructure needs to trace items regardless if they are hidden or blacklisted. It is now the responsibility of consumers of a traversal to filter out such items if they are unwanted. I did this by turning the `ir::context::WhitelistedItems` type alias into a proper struct that performs the filtering. This approach avoids changing the many callers of whitelisted items traversal iteration.

Additionally, and equally importantly, I realized that the case of template instantiations' template usages were subtly wrong. Before this commit, we only considered a template argument used if it itself was a template parameter. This breaks down in the face of template instantiations used as arguments to another template instantiation. If the inner instantiation used a template parameter, we would accidentally lose that usage. For example:

```c++
template <class T>
struct Foo {
    // Direct usage of a template parameter T in an instantiation always worked.
    Bar<T> member;

    // While indirect usage of a template parameter nested within another
    // instantiation did NOT previously work.
    Bar< Qux<T> > another_member;
};
```

The solution is to take the union of each template arguments' template usages, not the template arguments themselves but only if they were themselves a template parameter. Obvious in retrospect!

Fixes #662.

r? @emilio

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