[flang] Semantic checks for OpenMP combined constructs.

Summary:
This includes a refactor of the existing combined construct checks
that were present, as well as adding the remaining combined constructs
that had not been implemented yet.

Reviewers: jdoerfert

Subscribers: yaxunl, guansong, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77812