Function bindings are the only variables in LEGACY_CONST mode.
(https://codereview.chromium.org/1819123002/). Since these variables
can also be accessed in strict mode functions we should support
handling such variables. Assigning to a legacy constant throws
a TypeError in strict mode. Also fixes hydrogen.cc to throw a
TypeError for legacy constants.

BUG=v8:4280,chromium:599068
LOG=N
TBR=rmcilroy@chromium.org

Review URL: https://codereview.chromium.org/1845223006

Cr-Commit-Position: refs/heads/master@{#35383}