References to invalid names (i.e. not specified as a named group in the
pattern) throw a SyntaxError. Unmatched groups are still replaced by the
empty string.

See https://github.com/tc39/proposal-regexp-named-groups/issues/14.

BUG=v8:5437

Review-Url: https://codereview.chromium.org/2791183002
Cr-Commit-Position: refs/heads/master@{#44471}