Internals: Refactor AstCFunc internals (#6280) (#6578)

- Delete 'finalsp'. It was used in one place, basically unnecessary and
  safe to remove.
- Make 'argsp' a 'List[AstVar]'. This held before. It holds the function
  argument and return variables.
- Replace 'intitsp' with 'varsp' and make it into 'List[AstVar]' to hold
  the function local variables. This was most of its use before. The few
  places we inserted statements here now moved into 'stmtsp' by
  inserting at the front of the list.

src/V3Task.cpp

@@ -372,7 +372,7 @@ class TaskVisitor final : public VNVisitor {
     AstVarScope* createFuncVar(AstCFunc* funcp, const string& name, AstVar* examplep) {
         AstVar* const newvarp = new AstVar{funcp->fileline(), VVarType::BLOCKTEMP, name, examplep};
         newvarp->funcLocal(true);
-        funcp->addInitsp(newvarp);
+        funcp->addVarsp(newvarp);
         AstVarScope* const newvscp = new AstVarScope{funcp->fileline(), m_scopep, newvarp};
         m_scopep->addVarsp(newvscp);
         return newvscp;
@@ -1256,7 +1256,7 @@ class TaskVisitor final : public VNVisitor {
             // Need symbol table
             if (cfuncp->name() == "new") {
                 const string stmt = VIdProtect::protect("_ctor_var_reset") + "(vlSymsp);";
-                cfuncp->addInitsp(new AstCStmt{nodep->fileline(), stmt});
+                cfuncp->addStmtsp(new AstCStmt{nodep->fileline(), stmt});
             }
         }
         if (nodep->dpiContext()) {
@@ -1272,7 +1272,7 @@ class TaskVisitor final : public VNVisitor {
             // The AstScopeName is really a statement(ish) for tracking, not a function
             snp->dpiExport(true);
             snp->unlinkFrBack();
-            cfuncp->addInitsp(snp);
+            cfuncp->addStmtsp(snp);
         }
 
         // Create list of arguments and move to function
@@ -1332,7 +1332,7 @@ class TaskVisitor final : public VNVisitor {
 
         // Return statement
         if (rtnvscp && nodep->taskPublic()) {
-            cfuncp->addFinalsp(new AstCReturn{
+            cfuncp->addStmtsp(new AstCReturn{
                 rtnvscp->fileline(), new AstVarRef{rtnvscp->fileline(), rtnvscp, VAccess::READ}});
         }
         // Replace variable refs