Create function name const assignment after parsing language mode.

Create function name const assignment after parsing language mode.

Otherwise we may choose sloppy const or strict const depending on
whether the function is parsed the first time.

R=mvstanton@chromium.org
BUG=v8:4336
LOG=N

Review URL: https://codereview.chromium.org/1260053004

Cr-Commit-Position: refs/heads/master@{#29966}