This way we can ship \u{..} escapes in strings / identifiers before shipping /u
regexps.

BUG=

Review URL: https://codereview.chromium.org/903703002

Cr-Commit-Position: refs/heads/master@{#26461}

Allows \u{xxxxx} in regexps. Behind the --harmony-unicode flag.

Part 1 is here: https://codereview.chromium.org/716423002

BUG=v8:3648
LOG=N

Review URL: https://codereview.chromium.org/788043005

Cr-Commit-Position: refs/heads/master@{#26018}