Add Semantic check for Flang OpenMP 4.5 - 2.7.1 Do Loop restrictions.

Implementation of Do loop iteration variable check, Do while loop check, Do loop cycle restrictions.
Also to check whether the ordered clause is present on the loop construct if any ordered region ever
 binds to a loop region arising from the loop construct.

Files:

check-omp-structure.h
check-omp-structure.cpp
resolve-directives.cpp

Testcases:

omp-do06-positivecases.f90
omp-do06.f90
omp-do08.f90
omp-do09.f90
omp-do10.f90
omp-do11.f90
omp-do12.f90
omp-do13.f90
omp-do14.f90
omp-do15.f90
omp-do16.f90
omp-do17.f90

Reviewed by: Kiran Chandramohan @kiranchandramohan , Valentin Clement @clementval

Differential Revision: https://reviews.llvm.org/D92732