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/V3Timing.cpp

@@ -836,7 +836,14 @@ class TimingControlVisitor final : public VNVisitor {
 
         nodep->rtnType("VlCoroutine");
         // If in a class, create a shared pointer to 'this'
-        if (m_classp) nodep->addInitsp(new AstCStmt{nodep->fileline(), "VL_KEEP_THIS;"});
+        if (m_classp) {
+            AstCStmt* const cstmtp = new AstCStmt{nodep->fileline(), "VL_KEEP_THIS;"};
+            if (AstNode* const stmtsp = nodep->stmtsp()) {
+                stmtsp->addHereThisAsNext(cstmtp);
+            } else {
+                nodep->addStmtsp(cstmtp);
+            }
+        }
         AstNode* firstCoStmtp = nullptr;  // First co_* statement in the function
         nodep->exists([&](AstCAwait* const awaitp) -> bool { return (firstCoStmtp = awaitp); });
         if (!firstCoStmtp) {