[parser] disallow language mode directive in body of function with non-simple parameters

src/messages.h

Index: src/messages.h
diff --git a/src/messages.h b/src/messages.h
index f00ff8bc89c71e6251ef29ce1aaba368a59e1b63..942b68f3bacfc7b49456e990bcad712abd329b3a 100644
--- a/src/messages.h
+++ b/src/messages.h
@@ -296,6 +296,8 @@ class CallSite {
   T(IllegalAccess, "Illegal access")                                           \
   T(IllegalBreak, "Illegal break statement")                                   \
   T(IllegalContinue, "Illegal continue statement")                             \
+  T(IllegalLanguageModeDirective,                                              \
+    "Illegal '%' directive in function with non-simple parameter list")        \
   T(IllegalReturn, "Illegal return statement")                                 \
   T(InvalidLhsInAssignment, "Invalid left-hand side in assignment")            \
   T(InvalidLhsInFor, "Invalid left-hand side in for-loop")                     \