Auto merge of #54906 - qnighy:fix-issue-50452, r=nikomatsakis

Auto merge of #54906 - qnighy:fix-issue-50452, r=nikomatsakis

Reattach all grandchildren when constructing specialization graph.

Specialization graphs are constructed by incrementally adding impls in the order of declaration. If the impl being added has its specializations in the graph already, they should be reattached under the impl. However, the current implementation only reattaches the one found first. Therefore, in the following specialization graph,

```
  Tr1
   |
   I3
  /  \
 I1  I2
```

If `I1`, `I2`, and `I3` are declared in this order, the compiler mistakenly constructs the following graph:

```
  Tr1
  /  \
 I3  I2
  |
 I1
```

This patch fixes the reattach procedure to include all specializing grandchildren-to-be.

Fixes #50452.