Per http://tc39.github.io/ecma262/#sec-identifiers-static-semantics-early-errors (13.2.2),
make it a SyntaxError if an Identifier has the same StringValue as a ReservedWord.

BUG=v8:2222, v8:1972
LOG=N
R=adamk@chromium.org, rossberg@chromium.org, wingo@chromium.org

Review URL: https://codereview.chromium.org/1429983002

Cr-Commit-Position: refs/heads/master@{#32052}