[flang][OpenMP] Lowering critical construct

This patch adds translation from PFT to FIR for critical construct.

This is part of the upstreaming effort from the fir-dev branch in [1].
[1] https://github.com/flang-compiler/f18-llvm-project



Co-authored-by: kiranchandramohan <kiranchandramohan@gmail.com>

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D122218