An identifier may be parsed in an object literal like {let}, but
this was previously left out of lexical name checking. This patch
adds that check to prohibit code like
  let {let} = {let: 1}

BUG=v8:4403
LOG=N
R=adamk

Review URL: https://codereview.chromium.org/1401253003

Cr-Commit-Position: refs/heads/master@{#31278}