Rollup merge of #89098 - GuillaumeGomez:where-bounds-order, r=camelid

Rollup merge of #89098 - GuillaumeGomez:where-bounds-order, r=camelid

Fix generics where bounds order

Fixes #88809.

Like said on the above issue, the issue is that we were expecting `Symbol` comparisons to be string-based but they are integer-based (because `Symbol` is an integer), messing up the bounds order. To fix it, I simply stored into a `FxIndexMap` instead.

r? ``@jyn514``