This patch implements ES2015 RegExp subclassing semantics, namely the
hardest part where RegExp.prototype.exec and certain flag getters can
be overridden in order to provide different behavior. This change is
hidden behind a new flag, --harmony-regexp-exec. The flag guards the
behavior by installing entirely different implementations of the
methods which follow the new semantics.

Preliminary performance tests show a 3-4x regression in the Octane
RegExp benchmark. The new code doesn't call out into several fast
paths that the old code supported, so this is expected.

The patch is tested mostly by test262, where most RegExp tests are fixed,
with the exception of deliberate spec violations for web compatibility,
and for the 'sticky' flag, which is not dynamically read by this patch
in all cases but rather statically compiled into the RegExp. The latter
will require a follow-on patch to implement. A small additional set of
tests verifies one particular case, mostly to check whether the flag
mechanism works.

R=adamk,yangguo@chromium.org
LOG=Y
BUG=v8:4602

Review URL: https://codereview.chromium.org/1596483005

Cr-Commit-Position: refs/heads/master@{#35068}