[Flang][OpenMP][NFC][2/2] Reorder OmpStructureChecker and simplify it.

`OmpStructureChecker` has too much boilerplate code in source file.

This patch:
  1. Use helpers from `check-directive-structure.h` and reduces the boilerplate.
  2. Use TableGen infrastructure as much as possible.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D90834