[flang][openacc] Lower acc routine with function name

The routine directive can appear in the specification part of
a subroutine, function or module and therefore appear before the
function or subroutine is lowered. We keep track of the created
routine info attribute and attach them to the function at the end
of the lowering if the directive appeared before the function was
lowered.

Reviewed By: razvanlupusoru

Differential Revision: https://reviews.llvm.org/D158204