[base] Sync Optional implementation to chromium

There were a lot of tweaks and optimizations to chromium's
{base::Optional} implementation. This CL brings us back in sync with
that.
Some changes were needed to make this compatible with C++11 and with
GCC 4.8:
1) Types like std::decay_t and std::enable_if_t were rewritten to
   use std::decay and std::enable_if.
2) Some conditional no_except declarations were removed.
3) std::is_trivially_copy_constructible and
   std::is_trivially_move_constructible are assumed to be false on
   gcc 4 because it's unimplemented there.

R=neis@chromium.org

Bug: v8:8238
Change-Id: Ia0542c0d4d2fd43a2454f639ec5201ad8d8201cd
Reviewed-on: https://chromium-review.googlesource.com/c/1275824
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56625}