[Flang] Lower the sqrt intrinsics

[Flang] Lower the sqrt intrinsics

The intrinsic computes the square root for real and complex numbers. By
default they are lowered to runtime calls to libpgmath. With the llvm
option, it can be lowered to llvm intrinsics (not all types .eg. complex
are supported for llvm lowering).

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: schweitz

Differential Revision: https://reviews.llvm.org/D122018

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