Fix line nums when multiline ${expr} spans here-doc

Fix line nums when multiline ${expr} spans here-doc

<<end . ${

end
"bar"};
warn __LINE__ # 3, not 5

This was caused by commit a49b10d0a, which make scan_ident in toke.c
reallocate the parser’s current line buffer (SvPVX(PL_linestr)) to
search for whitespace surrounding an identifier.

In case there is an arbitrary expression, it temporarily records the
line number and resets it at the end if that turns out to be the case.
However, it was not resetting PL_parser->herelines, which records how
many line numbers to skip when next incrementing it (to skip past
here-doc bodies).

So save and restore that value, too.