[flang] use hlfir base when translating assumed-rank entity to fir::ExtendedValue (#94822)

The hlfir::Entity to fir::ExtendedValue conversion usually uses the "fir
base" output of hlfir.declare (which is the same as the input) to avoid
introducing temporary descriptors for the sole purpose of introducing
updating lower bound information. This is possible because local lower
bounds, if any, are tracked in a vector inside the fir::ExtendedValue.

With assumed-ranks, the lower bounds cannot be tracked inside the
fir::ExtendedValue vector (their numbers is unknown at compile time).
Hence, the fir.box/fir.class used in fir::ExtendedValue in lowering must
always contain accurate local lower bound information.