add test for issue 86507

add stderr for issue 86507

update issue-86507 UI test

add comment for the expected error in UI test file

add proper 'refers to <ref_type>' in suggestion

update diagnostic phrasing; update test to match new phrasing; re-organize logic for checking T: Sync

evaluate additional obligation to figure out if T is Sync

run './x.py test tidy --bless'

incorporate changes from review; reorganize logic for readability

the new diagnostic messages