[flang][hlfir] Support fir.declare in AbstractResult pass

The AbstractResult pass replaces allocation of function result on the callee
side per an extra argument so that the allocation of the result can be
done on the caller stack.
It looks for the result allocation from the fir.return op, so it needs
to handle (in a transparent way) a fir.declare in the chain between the
allocation and the fir.return.

Reviewed By: vzakhari, clementval

Differential Revision: https://reviews.llvm.org/D149057