[flang] Lower symbols to hlfir.declare

Update lowering to generate hlfir.declare instead of fir.declare.
Introduce the hlfir::Entity class that will be used to work with
Fortran objects in HLFIR transformation.

Fix lower bounds that where swapped with extents in fir.declare
generation.

Update tests that expected fir.declare.

Differential Revision: https://reviews.llvm.org/D137951