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

Add Semantic check for Flang OpenMP 4.5 - 2.7.1  Do Loop restrictions on single directive and firstprivate clause.

Semantic checks added to check the worksharing 'single' region closely nested inside a worksharing 'do' region. And also to check whether the 'do' iteration variable is a variable in 'Firstprivate' clause.

Files:
check-directive-structure.h
check-omp-structure.h
check-omp-structure.cpp

Testcases:
omp-do01-positivecase.f90
omp-do01.f90
omp-do05-positivecase.f90
omp-do05.f90

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

Differential Revision: https://reviews.llvm.org/D93205