Fix unintended fall-through.

Unfortunately I've not found a way to exercise this code that doesn't
crash elsewhere yet, due to unrelated bugs in how Sema incorrectly
instantiates lambdas in function template signatures.

clang/lib/AST/ItaniumMangle.cpp

@@ -4972,6 +4972,7 @@ void CXXNameMangler::mangleExpression(const Expr *E, unsigned Arity,
     Out << "L";
     mangleType(Context.getASTContext().getRecordType(cast<LambdaExpr>(E)->getLambdaClass()));
     Out << "E";
+    break;
   }
 
   case Expr::PackExpansionExprClass: