Note this isn't implementing checking through imports. The parse node
there is harder to access through the context, so would require
examining the entity in order to get the import declaration, to get at
the ImportIR. We also don't have a parse tree attached in that case, and
would need to add one to SemIR::File. But I believe we do want to add
that, so it's explicitly a TODO.
Note GetTokenText re-lexes literal values, so there's a bit of potential
overhead there. Not sure if we want a more efficient manner for
comparing in cases like this.
Creates a `GlobalInit` class for storing relevant values, pulling
functions off `InstBlockStack` and `Context`. Adds a `Context` pointer
just so that it doesn't need to be passed in on each call (`Finalize` in
particular uses several members).
Note we have several different `InstBlockStack` instances, so several
copies of the relevant members were simply unused.
Jon Ross-Perkins