Fix control-flow handling of modifiers without body.

libsolidity/analysis/ControlFlowBuilder.cpp

@@ -308,7 +308,7 @@ bool ControlFlowBuilder::visit(ModifierInvocation const& _modifierInvocation)
 		_modifierInvocation.name().annotation().referencedDeclaration
 	);
 	if (!modifierDefinition) return false;
-	solAssert(!!modifierDefinition, "");
+	if (!modifierDefinition->isImplemented()) return false;
 	solAssert(!!m_returnNode, "");
 
 	m_placeholderEntry = newLabel();

libsolidity/analysis/ControlFlowBuilder.h

@@ -29,9 +29,8 @@
 namespace solidity::frontend
 {
 
-/** Helper class that builds the control flow of a function or modifier.
- * Modifiers are not yet applied to the functions. This is done in a second
- * step in the CFG class.
+/**
+ * Helper class that builds the control flow of a function or modifier.
  */
 class ControlFlowBuilder: private ASTConstVisitor, private yul::ASTWalker
 {