[Flang][OpenMP] Avoid double privatisation of loop variables

Loop variables of a worksharing loop and sequential loops in parallel
region are privatised by default. These variables are marked with
OmpPreDetermined. Skip explicit privatisation of these variables.

Note: This is part of upstreaming from the fir-dev branch of
https://github.com/flang-compiler/f18-llvm-project.

Reviewed By: Leporacanthicus

Differential Revision: https://reviews.llvm.org/D127249



Co-authored-by: Jean Perier <jperier@nvidia.com>
Co-authored-by: Mats Petersson <mats.petersson@arm.com>