Split V3Order into further part and decouple various components (#4953)

Continuing the idea of decoupling the implementations of the various algorithms.

The main points:

-Move the former "processDomain" stuff, dealing with assigning combinational logic into the relevant sensitivity domains into V3OrderProcessDomains.cpp

-Move the parallel code construction in V3OrderParallel.cpp (Could combine this with some parts of V3Partition - those not called from V3Partition::finalize - but that's not for this patch).

-Move the serial code construction into V3OrderSerial.cpp

-Factored the very small common code between the parallel and serial code construction (processMoveOneLogic) into V3OrderCFuncEmitter.cpp

src/V3SchedPartition.cpp

@@ -185,7 +185,6 @@ class SchedGraphBuilder final : public VNVisitor {
         // Clocked or hybrid logic has explicit sensitivity, so add edge from sensitivity vertex
         if (!m_senTreep->hasCombo()) {
             m_senTreep->foreach([this, nodep, logicVtxp](AstSenItem* senItemp) {
-                if (senItemp->isIllegal()) return;
                 UASSERT_OBJ(senItemp->isClocked() || senItemp->isHybrid(), nodep,
                             "Non-clocked SenItem under clocked SenTree");
                 V3GraphVertex* const eventVtxp = getSenVertex(senItemp);