3905: add ellipsis field to hir pat record r=matklad a=JoshMcguigan
This PR corrects a `fixme`, adding an `ellipsis` field to the hir `Pat::Record` type. It will also be unlock some useful follow on work for #3894.
Additionally it adds a diagnostic for missing fields in record patterns.
~~Marking as a draft because I don't have any tests, and a small amount of manual testing on my branch from #3894 suggests it might *not* be working. Any thoughts on how I can best test this, or else pointers on where I might be going wrong?~~
Co-authored-by: Josh Mcguigan <joshmcg88@gmail.com>
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