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

[fabianschuiki]

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.