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/V3Premit.cpp

@@ -91,7 +91,7 @@ class PremitVisitor final : public VNVisitor {
             // Keep as local temporary.
             const std::string name = "__Vtemp_" + std::to_string(++m_tmpVarCnt);
             varp = new AstVar{flp, VVarType::STMTTEMP, name, nodep->dtypep()};
-            m_cfuncp->addInitsp(varp);
+            m_cfuncp->addVarsp(varp);
             ++m_temporaryVarsCreated;
 
             // Assignment to put before the referencing statement