pub unsafe fn set_filled(
        &mut self,
        builder: LLVMBuilderRef,
        slot: LLVMValueRef,
        value: LLVMValueRef,
    ) {
        let filled_pointer = LLVMBuildStructGEP(builder, slot, FILLED_INDEX, c_str!(""));
        LLVMBuildStore(builder, value, filled_pointer);
    }
Set the filled value to a value between 1 and 255.

The filled value represents the capacity of a grouping vector in a grouping dictionary. A
nonzero filled value indicates an initialized slot.

NOTE: It is invalid to "unfill" a slot by changing it from a nonzero value to a zero value.
We could add a debug assertion here to check that this never happens.
weld/src/codegen/llvm2/dict.rs