make `PlaceholderConst` not store the type of the const
Currently the `Placeholder` variant on `ConstKind` is 28 bytes when with this PR its 8 bytes, i am not sure this is really useful at all rn since `Unevaluated` and `Value` variants are huge still but eventually it should be possible to get both down to 16 bytes 🤔. Mostly opening this to see if this change has any perf impact when done before it can make `ConstKind`/`ConstS` smaller
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| .. | ||
| chalk | ||
| dropck_outlives.rs | ||
| evaluate_obligation.rs | ||
| implied_outlives_bounds.rs | ||
| lib.rs | ||
| normalize_erasing_regions.rs | ||
| normalize_projection_ty.rs | ||
| type_op.rs | ||