add tentative 5.18.2 release date
PATCH [perl #119713] Regex \8 and \9 after literals no longer work

Commit 726ee55d introduced a regression that has been fixed in blead by
commit f1e1b256.  However the later commit changed some buggy behavior
into errors instead of warnings, and so is contraindicated in a
maintenance release.  This current commit attempts to fix the regression
without changing other behavior.  It includes the pat.t tests from f1e1b256.