Fix LIKE matching with nondeterministic collations and backslashes.

Fix LIKE matching with nondeterministic collations and backslashes.

Commit 85b7efa1c added support for LIKE with nondeterministic
collations, but it included a bug in the de-escaping logic for
literal pattern substrings.  That unconditionally skipped all
backslashes, but when it encounters '\\' it should emit the second
backslash as a de-escaped character.  That led to acting as though
the escaped backslash was not there.

Bug: #19474
Reported-by: Bowen Shi <zxwsbg12138@gmail.com>
Author: Nitin Motiani <nitinmotiani@google.com>
Reviewed-by: Zsolt Parragi <zsolt.parragi@percona.com>
Reviewed-by: Ewan Young <kdbase.hack@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/19474-5b86a95f3d9a7ecb@postgresql.org
Discussion: https://postgr.es/m/CAH5HC94yU+K8Gcdy12M5BS8gwD_SXLSHzc9k5tNk7JDnpBiFMA@mail.gmail.com
Backpatch-through: 18