cxxrtl: Prevent wires with input or output ports from being aliased

[noeamiot]

As explained in #4405, having outputs or inputs aliased causes a loss of information in debug_info.

To prevent this behavior, we stop assigning debug information at the beginning of the debug_alias option corresponding logic if the wire is an input and/or an output.

One implementation choice here was to choose, for an input that is tied to a constant, whether we should generate:

items->add(path, "a0", "src\000sand_ti.v:1.18-1.20\000", p_a0, 0, debug_item::INPUT|debug_item::DRIVEN_COMB);

or

static const value<1> const_p_a0 = value<1>{0u};
items->add(path, "a0", "src\000sand_ti.v:1.18-1.20\000", const_p_a0);

This patch is an implementation for the first one, because we can still query the constant later in the first whereas we would loose the INPUT and DRIVEN_COMB flags in the second one.

[whitequark]

LGTM, thanks!