Fix let pattern error accumulation

When the checker was added prohibiting lexical binding called let,
certain error propagation was not implemented properly. This patch
fixes that issue, which fixes error checking for cases such as
  let [let]

BUG=v8:4403
R=adamk
LOG=N

Review URL: https://codereview.chromium.org/1409613003

Cr-Commit-Position: refs/heads/master@{#31289}

src/expression-classifier.h

@@ -264,6 +264,8 @@ class ExpressionClassifier {
       if (errors & StrongModeFormalParametersProduction)
         strong_mode_formal_parameter_error_ =
             inner.strong_mode_formal_parameter_error_;
+      if (errors & LetPatternProduction)
+        let_pattern_error_ = inner.let_pattern_error_;
     }
 
     // As an exception to the above, the result continues to be a valid arrow

test/message/let-lexical-name-in-array-prohibited.js

+// Copyright 2015 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+//
+// Flags: --harmony-sloppy --harmony-sloppy-let --harmony-destructuring
+
+let [let];

test/message/let-lexical-name-in-array-prohibited.out

+*%(basename)s:7: SyntaxError: let is disallowed as a lexically bound name
+let [let];
+     ^^^
+SyntaxError: let is disallowed as a lexically bound name
+