For web compat reasons, we support an initializer in the declaration
part of a for-in loop. But we should disallow this for destructured
declarations (just as we do for lexical declarations). In fact, without
disallowing it, we crash.

Also fix up the PreParser to have the same restrictions here as the parser
(the lexical check was missing there), verified by running the message tests
with --min-preparse-length=0.

In fixing the logic I've also cleaned up the code a bit, removing the
only-called-once DeclarationParsingResult::SingleName method.

BUG=v8:811
LOG=n

Review URL: https://codereview.chromium.org/1471973003

Cr-Commit-Position: refs/heads/master@{#32236}