Internals: Use and enforce class final for ~5% performance boost.

src/V3Descope.cpp

@@ -34,7 +34,7 @@
 
 //######################################################################
 
-class DescopeVisitor : public AstNVisitor {
+class DescopeVisitor final : public AstNVisitor {
 private:
     // NODE STATE
     //  Cleared entire netlist