base::Optional is a replacement for std::optional, until we switch to
C++17 and can use std::optional directly.
The implementation is copied from chromium's base::Optional, but put in
the {v8::base} namespace instead of just {base}. Also, the
specialization of std::hash for base::Optional is omitted, since it's
disallowed in the style guide.

A first use in the AsmJsParser is introduced, if that one sticks, I
will refactor more uses of std::unique_ptr to use base::Optional
instead, avoiding the heap allocation.

R=mstarzinger@chromium.org
BUG=v8:6474

Change-Id: I019599d4bf9ff0105bf592dfb96d6050feba18ae
Reviewed-on: https://chromium-review.googlesource.com/528884
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45810}