Rollup merge of #117262 - celinval:issue-38-norm, r=oli-obk

Create a new ConstantKind variant (ZeroSized) for StableMIR

ZeroSized constants can be represented as `mir::Const::Val` even if their layout is not yet known. In those cases, CrateItem::body() was crashing when trying to convert a `ConstValue::ZeroSized` into its stable counterpart  `ConstantKind::Allocated`.

Instead, we now map `ConstValue::ZeroSized` into a new variant: `ConstantKind::ZeroSized`.

**Note:** I didn't add any new test here since we already have covering tests in our project repository which I manually confirmed that will fix the issue.

Author: matthiaskrgr