[Flang][OpenMP] Initial lowering of the OpenMP worksharing loop

The OpenMP worksharing loop operation in the dialect is a proper loop
operation and not a container of a loop. So we have to lower the
parse-tree OpenMP loop construct and the do-loop inside the construct
to a omp.wsloop operation and there should not be a fir.do_loop inside
it. This is achieved by skipping fir.do_loop creation and calling genFIR
for the nested evaluations in the lowering of the do construct.

Note: Handling of more clauses, parallel do, storage of loop index variable etc will come in separate patches.

Part of the upstreaming effort to move LLVM Flang from fir-dev branch of
https://github.com/flang-compiler/f18-llvm-project to the LLVM Project.

Reviewed By: peixin

Differential Revision: https://reviews.llvm.org/D125024



Co-authored-by: Sourabh Singh Tomar <SourabhSingh.Tomar@amd.com>
Co-authored-by: Shraiysh Vaishay <Shraiysh.Vaishay@amd.com>