[Flang] enable fir.is_present and fir.absent with function types

Fortran dummy procedures and procedure pointer can be OPTIONAL, and
there is no technical reason to prevent fir.is_present and
fir.absent from accepting function types, so allow it and add test.

Note: This is part of upstreaming from the fir-dev branch of
https://github.com/flang-compiler/f18-llvm-project. This patch is
basically upstreaming the following PR.
https://github.com/flang-compiler/f18-llvm-project/pull/1568

Reviewed By: clementval

Differential Revision: https://reviews.llvm.org/D128464



Co-authored-by: Jean Perier <jperier@nvidia.com>