The catch variable is a special VAR-mode variable which is not in a declaration
scope. Normally creating such a variable is not possible with DeclareVariable,
but Parser bypasses it by calling DeclareLocal directly (which doesn't have the
hoisting check).

PreParser used to cut corners and declare the catch variable as a LET-mode
variable to prevent hoisting.

But since LET and VAR variables behave differently when deciding whether they
block sloppy block function hoisting, that approach doesn't fly.

BUG=v8:5516,chromium:771474

Change-Id: Ic6f5f4996416c9fa59132725c8b0b6b570c72f48
Reviewed-on: https://chromium-review.googlesource.com/700634
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48308}