Rollup merge of #39595 - camlorn:structured_repr, r=eddyb

Make reprs use a structured representation instead of a slice

This is needed for `-z reorder-fields`.  The old design uses a slice taken from HIR, plus a cache that lazily parses.  The new design stores it directly in the `AdtDef` as a `ReprOptions`.  We're doing this now because we need to be able to add reprs that don't necessarily exist in HIR for `-z reorder-fields`, but it needs to happen anyway.

`lookup_repr_hints` should be mostly deprecated.  I want to remove it from `layout` before closing this, unless people think that should be a separate PR.  The `[WIP]` is because of this.  The problem with closing this as-is is that the code here isn't actually testable until some parts of the compiler start using it.

r? @eddyb

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