much better swap logic to support reentrancy and fix assert failure

much better swap logic to support reentrancy and fix assert failure

Commit c74340f9 added backreferences as well as the idea of a ->swap
regex pointer to keep track of the match offsets in case of backtracking.
The problem is that when Perl re-enters the regex engine to handle
utf8::SWASHNEW, the ->swap is not saved/restored/cleared so any capture
from the utf8 (Perl) code could inadvertently modify the regex match
data that caused the utf8 swash to get built.

This change should close out RT #60508