Support "Unroll" Loop Control via function-scoped #[spirv(unroll_loops)].

[eddyb]

This required generalizing the zombie decoration system (see the first commit), because OpLoopMerge (where the Unroll "Loop Control" has to be applied) are created during linking, by the structurizer, not in the original crate (which is the one understanding the mapping between SPIR-V OpFunctions and Rust fns).

It's not as nice as I would've hoped, but because of all the RAUW (Replace All Uses With) happening during linking, we can't just decode all of the decorations eagerly and keep them in e.g. a HashMap, as the IDs being decorated by them won't be replaced by RAUW.

So instead of centralizing such rustc_codegen_spirv decorations into a single enum and HashMap, each type that implements the CustomDecoration trait has to be independently decoded and removed from the SPIR-V module.