[flang][openacc] Lower acc declare to the new acc.declare function

Lower the acc delcare directive in function/subroutine
to the newly introduced acc.declare operation. Only a single
acc.declare operation is procduced in a function or subroutine
so they don't end up nested.

Depends on D158314

Reviewed By: razvanlupusoru

Differential Revision: https://reviews.llvm.org/D158315