Merge #3894

3894: Match check enum record r=flodiebold a=JoshMcguigan

This PR implements match statement exhaustiveness checking for record type enums.

It also make a minor addition to the test infrastructure to allow testing against a single diagnostic, so you can be sure your test is triggering (or not) whichever diagnostic you expect.

Co-authored-by: Josh Mcguigan <[email protected]>

Author: bors[bot]