reset pos and utf8 cache when de/encoding utf8 str

reset pos and utf8 cache when de/encoding utf8 str

When using
    utf8::upgrade
    utf8::downgrade
    utf8::encode
    utf8::decode
or the underlying C-level functions
    sv_utf8_upgrade_flags_grow
    sv_utf8_downgrade
    sv_utf8_encode
    sv_utf8_decode
and
    sv_recode_to_utf8

update the position of the pos magic, if any, and clear the utf8
length/position-mapping cache.

This fixes [perl #80190].