[LLHD] Switch from hw.inout to a custom signal reference type

A while ago, we have switched the LLHD dialect from a custom !llhd.sig wrapper type to !hw.inout in order to represent a reference to a signal slot that can be probed and driven. To support Verilog's time values, we have to be able to store an !llhd.time value in a variable and read and write that variable. Variables are effectively signals, and Verilog variables get mapped to llhd.sig allocation operations. These return a reference type. This prevents us from creating time variables today, since !hw.inout forces isHWValueType on the inner type, which llhd::TimeType is not.

Ideally, we'd create something like an !llhd.ref<T> type to represent a reference to a signal. In the future, we might want to add additional types for multi-driver nets.

Start by tracing the LLHD dialect's use of !hw.inout and the llhd.sig allocation operations described in the issue. Review how signal references are represented and how !llhd.time variables would need to be stored and read; done means replacing this representation with a custom reference type that supports non-HW inner types.