harmony-scoping: make assignment to 'const' a late error.

Per TC39 Nov 2014 decision.

This patch also changes behavior for "legacy const": assignments to sloppy const in strict mode is now also a type error. This fixes v8:2243 and also brings us in compliance with other engines re assignment to function names (see updated webkit test), but might have bigger implications.
That change can easily be reverted by changing Variable::IsSignallingAssignmentToConst.

BUG=v8:3713,v8:2243
LOG=N

Review URL: https://codereview.chromium.org/749633002

Cr-Commit-Position: refs/heads/master@{#25516}