This moves the RegExp constructor to C++. Local runs of octane/regexp are
performance-neutral:

C++: 4970.1 +- 404.981
JS: 4869.2 +- 586.743

That's probably only the case because exec and replace dominate
octane/regexp. There's potential for improvement here, for instance by
adding a fast-path if new.target is an unmodified JSRegExp function.

BUG=v8:5339

Review-Url: https://codereview.chromium.org/2384613004
Cr-Commit-Position: refs/heads/master@{#39981}