[flang][OpenMP] Only use HLFIR base in privatization logic (#84123)

Modifies the privatization logic so that the emitted code only used the
HLFIR base (i.e. SSA value `#0` returned from `hlfir.declare`). Before
that, that emitted privatization logic was a mix of using `#0` and `#1`
which leads to some difficulties trying to move to delayed privatization
(see the discussion on #84033).