Also fix CheckConflictingVarDeclarations() to properly handle
legacy const bindings. Without that change enabling the flag
causes code like:

  function f() { const x; var x; }

to throw an early error, rather than wait to throw the error
until f is invoked.

BUG=v8:811
LOG=y
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_chromium_rel_ng;tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1451843002

Cr-Commit-Position: refs/heads/master@{#32222}