Split off a separate --harmony_sloppy_let flag

Split off a separate --harmony_sloppy_let flag

--harmony_sloppy includes behavior to turn on sloppy mode lexical
bindings. Before this patch, it also included a way to parse let
which is likely web-incompatible (let is disallowed as an
identifier). This patch splits off the let parsing from the more
general block scoping code, so that block scoping can be developed
independently.

R=adamk
LOG=N
BUG=v8:3305
Committed: https://crrev.com/2d2b72f6382df11056fb9c39da0581be85510459
Cr-Commit-Position: refs/heads/master@{#29855}

[adamk, 2015-07-24 22:36:10]

lgtm

[Dan Ehrenberg, 2015-07-24 23:18:20]

The CQ bit was checked by littledan@chromium.org

[Dan Ehrenberg, 2015-07-24 23:18:20]

The patchset sent to the CQ was uploaded after l-g-t-m from adamk@chromium.org
Link to the patchset: https://codereview.chromium.org/1255013002/#ps20001
(title: "Fix flags on mjsunit tests")

[commit-bot: I haz the power, 2015-07-24 23:18:33]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1255013002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1255013002/20001

[commit-bot: I haz the power, 2015-07-25 00:05:14]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-07-25 00:05:24]

Patchset 2 (id:??) landed as
https://crrev.com/2d2b72f6382df11056fb9c39da0581be85510459
Cr-Commit-Position: refs/heads/master@{#29855}