[WIP] Add existential structs to Checked C

[secure-sw-dev-bot]

This is a placeholder issue for a PR at checkedc/checkedc-clang#683. It contains only comments. For full details, see the original PR.

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Design doc: https://github.com/microsoft/checkedc/wiki/%5BProposal%5D-Add-Existential-Structs-to-CheckedC

PR with tests: checkedc/checkedc#382

I'll update the PR to be more meaningful when we're closing to merging this.

Current state of the PR: there are lots of TODOs, so it'd be helpful to get feedback both on the current state of things and prioritization of issues.

[secure-sw-dev-bot]

Comment from @abeln:

@dtarditi I've addressed most of your comments. PTAL. I've added a couple of questions.
I also updated the companion PR with more tests: checkedc/checkedc#382

I'm tracking the TODOs in the master issues for existentials (checkedc/checkedc-clang#661) and generics (checkedc/checkedc-clang#644).

[secure-sw-dev-bot]

Comment from @abeln:

@dtarditi the build succeeds now
https://dev.azure.com/msresearch/CheckedC/_build/results?buildId=342238