[flang][lowering] Add lowering for the `set_exponent` intrinsic

This patch adds support for lowering of the `set_exponent` intrinsic
from Fortran to the FIR dialect of MLIR.

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/D121805



Co-authored-by: Jean Perier <jperier@nvidia.com>
Co-authored-by: Eun Jung Park <ejpark@lanl.gov>