[flang][OpenMP] Upstream lowering of `ParallelOp` clauses

Note: This patch reflects the work that can be upstreamed from PR's(merged):

1. https://github.com/flang-compiler/f18-llvm-project/pull/456
2. https://github.com/flang-compiler/f18-llvm-project/pull/485

Also replaced TODO with new TODO.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D89769