This patch prohibits lexical bindings from being called 'let', even in
sloppy mode, following the ES2015 specification. The change affects
multiple cases of lexical bindings, including simple let/const declarations
and both kinds of for loops. var and legacy const bindings still permit
the name to be let, including in destructuring cases. Tests are added to
verify, though some cases are commented out since they led to (pre-existing)
crashes.

BUG=v8:4403
R=adamk
LOG=Y

Review URL: https://codereview.chromium.org/1371263003

Cr-Commit-Position: refs/heads/master@{#31115}