Auto merge of #52546 - nikomatsakis:issue-52050, r=pnkfelix

Auto merge of #52546 - nikomatsakis:issue-52050, r=pnkfelix

do not overwrite child def-id in place but rather remove/insert

When inserting a node N into the tree of impls, we sometimes find than an existing node C should be replaced with N. We used to overwrite C in place with the new def-id N -- but since the lists of def-ids are separated by simplified type, that could lead to N being inserted in the wrong place. This meant we might miss conflicts. We are now not trying to be so smart -- we remove C and then add N later.

Fixes #52050

r? @aturon -- do you still remember this code at all? :)