Enable --harmony-string-matchall by default.

String.prototype.matchAll behaves similarly to
String.prototype.match, but returns a full regexp
result object for each match in a global or sticky
regexp. This offers a simple way to iterate over
matches when access to e.g. capture groups is
needed.

const string = 'a b c';
const regex = /[ac]/g;
for (const match of string.matchAll(regex)) {
  console.log(`${match[0]} at ${match.index}`);
}
// a at 0
// c at 4

More information can be found here:
https://github.com/tc39/proposal-string-matchall

Drive-by: Update debug evaluate side effect
expectations to handle String.p.matchAll and
RegExp.p[@@matchAll]

Bug: v8:6890
Change-Id: Ie3e712af66689936b7d2a15df705b792ccf06bd3
Reviewed-on: https://chromium-review.googlesource.com/c/1377774Reviewed-by: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Peter Wong <peter.wm.wong@gmail.com>
Cr-Commit-Position: refs/heads/master@{#58250}