[flang] Add hlfir.declare codegen

hlfir.declare codegen generates a fir.declare, and may generate a
fir.embox/fir.rebox/fir.emboxchar if the base value does not convey
all the variable bounds and length parameter information.

Leave OPTIONAL as a TODO to keep this patch simple. It will require
making the embox/rebox optional to preserve the optionality aspects.

Differential Revision: https://reviews.llvm.org/D137789