Auto merge of #140923 - Zalathar:operand-bundle, r=lcnr

cg_llvm: Rename `OperandBundleOwned` to `OperandBundleBox`

As with `DIBuilderBox`, the "Box" suffix does a better job of communicating that this is an owning pointer to some borrowable resource.

This also renames the `raw` method to `as_ref`, which is what it would have been named originally if the `Deref` problem (#137603) had been known at the time.

No functional change.

Author: bors