[Flang] Lower the spacing, rrspacing intrinsics
These intrinsics returns the distance to the nearest real number and
their reciprocal. They are lowered to flang runtime calls.
This is part of the upstreaming effort from the fir-dev branch in [1].
[1] https://github.com/flang-compiler/f18-llvm-project
Reviewed By: clementval
Differential Revision: https://reviews.llvm.org/D121832
Co-authored-by: Mark Leair <leairmark@gmail.com>
Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>
Co-authored-by: Jean Perier <jperier@nvidia.com>