[Flang][OpenMP] Add implementation of privatisation

Privatisation creates local copies of variables in the OpenMP region.
Two functions `createHostAssociateVarClone` and `copyHostAssociateVar`
are added to create a clone of the variable for basic privatisation and to
copy the contents for first-privatisation.

Note: Tests for more data-types will be added when the fir.do_loop is
upstreamed.

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: peixin, NimishMishra

Differential Revision: https://reviews.llvm.org/D122595



Co-authored-by: Jean Perier <jperier@nvidia.com>
Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>
Co-authored-by: Peter Klausler <pklausler@nvidia.com>
Co-authored-by: Valentin Clement <clementval@gmail.com>
Co-authored-by: Sourabh Singh Tomar <SourabhSingh.Tomar@amd.com>
Co-authored-by: Nimish Mishra <neelam.nimish@gmail.com>
Co-authored-by: Peixin-Qiao <qiaopeixin@huawei.com>