[flang][lowering] Add support for lowering of the `{l|u}bound` intrinsics

This patch adds support for lowering of the `{l|u}bound` intrinsics from
Fortran to the FIR dialect of MLIR. Note that `ubound` is already
supported, but the test was missing (added here).

This is part of the upstreaming effort from the `fir-dev` branch in [1].

[1] https://github.com/flang-compiler/f18-llvm-project

Differential Revision: https://reviews.llvm.org/D121926



Co-authored-by: Jean Perier <jperier@nvidia.com>
Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>
Co-authored-by: Peter Steinfeld <psteinfeld@nvidia.com>