Ensure that every item we `constrain` has a set of used template parameters

This is a follow up to c8a206a, and the support for blacklisting in the named
template parameter usage analysis. This ensures that ever item we ever call
`constrain` on has an entry in `used` for the set of template parameters it
uses. Additionally, it adds extra assertions to enforce the invariant.

We cannot completely avoid analyzing blacklisted items because we want to
consider all of a blacklisted template's parameters as used. This is why we
ensure that blacklisted items have a used template parameter set rather than
ensuring that blacklisted items never end up in the worklist.

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