Auto merge of #56460 - davidtwco:issue-55850, r=pnkfelix

Auto merge of #56460 - davidtwco:issue-55850, r=pnkfelix

Fix ICE with generators and NLL

Fix #55850.

This PR stops an ICE in #55850 by not panicking when a region cannot be named. However, this PR does not (yet) fix the underlying issue that the correct name for the test case provided for the issue (in this instance, `'a`) was not found.

This PR also lays a little bit of groundwork by categorizing yields separately from returns so that region naming can be specialized for this case.

r? @pnkfelix