Batch of checked header changes from CCI 2021-05

[mattmccutchen-cci]

As previously discussed with Sulekha, CCI is preparing to submit a batch of changes to the checked headers that we made in support of our recent porting efforts. I'm going ahead and submitting a draft PR so that I can more easily refer to some of our changes in issues that I'm about to file. Warning: I may still overwrite the commits of the PR via force-push.

One part of the PR that I believe is in final form and that you can go ahead and review if you wish is the change to make #include-ing a wrapper header an error if the original header does not exist on the system. This change is currently in this commit (though I may overwrite the commit later). It might make sense to make this change into a separate PR (at the cost of some additional overhead) because it is different in nature from all our other changes; let me know if you'd like me to do so.

The corresponding draft PR for the tests in the checkedc-clang repository is microsoft/checkedc-clang#1064.