ES#sec-islabelledfunction specifies that labelled function declarations
may not occur as the body of a control flow construct such as an if
statement. This patch implements those restrictions, which also
eliminates a previous case resulting in a DCHECK failure which is now
a SyntaxError.

BUG=chromium:595309
R=adamk
LOG=Y

Review URL: https://codereview.chromium.org/1808373003

Cr-Commit-Position: refs/heads/master@{#35049}