[Flang][OpenMP] Add semantic checks for OpenMP Workshare Construct

Add Semantic checks for OpenMP 4.5 - 2.7.4 Workshare Construct.

 - The structured block in a workshare construct may consist of only
   scalar or array assignments, forall or where statements,
   forall, where, atomic, critical or parallel constructs.

 - All array assignments, scalar assignments, and masked array
   assignments must be intrinsic assignments.

 - The construct must not contain any user defined function calls unless
   the function is ELEMENTAL.

Test cases : omp-workshare03.f90, omp-workshare04.f90, omp-workshare05.f90

Resolve test cases (omp-workshare01.f90 and omp-workshare02.f90) marked as XFAIL

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D93091