In #22620 `diagnostic_group` was modified to return results for
multibuffers, but was returning singleton buffer points. `hover_popover`
uses it to find the jump target for clicking the popup - which doesn't
seem to be working right now but that's a separate issue. Now that
`diagnostic_group` is returning values in multibuffers converting these
to anchors was crashing.
Also resolves a potential bug - if folding in multibuffers was supported
then "Go To Diagnostics" would not properly skip diagnostics from folded
regions.
Release Notes:
- N/A
570e6c80a8