RT#69616: regexp SVs lose regexpness in assignment

RT#69616: regexp SVs lose regexpness in assignment

It uses reg_temp_copy to copy the REGEXP onto the destination SV without
needing to copy the underlying pattern structure. This means changing
the prototype of reg_temp_copy, so it can copy onto a passed-in SV, but
it isn't API (and probably shouldn't be exported) so I don't think this
is a problem.