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.

The previous patch ran into problems with the fuzzer; that crash was fixed
(with test coverage added) in https://crrev.com/ceb92ebfdfb561d71038793c02b42aa973f55ec4

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

Review URL: https://codereview.chromium.org/1473243006

Cr-Commit-Position: refs/heads/master@{#32306}