Revert "pos in lvalue context now returns a PVMG instead of a PVLV."

Revert "pos in lvalue context now returns a PVMG instead of a PVLV."

This reverts commit 571f0e8653a532c34edde36e797ecba446978b1c.

I’m afraid I have to revert this, as it does not modify sv_reftype
accordingly, and doing so would add *more* complexity (the opposite
of what that commit was trying to achieve) and slow down ref() at run
time, by making it search for pos magic.