Previously, named captures (and related functionality) were restricted to
unicode-mode regexps.

This CL extends that support to non-unicode patterns. Named groups are
supported regardless of the mode, and named back-references are supported if
the regexp is in unicode mode or if it contains a named capture (otherwise '\k'
is treated as an identity escape).

BUG=v8:5437,v8:6192

Review-Url: https://codereview.chromium.org/2788873002
Cr-Commit-Position: refs/heads/master@{#44324}