[Flang][OpenMP][Lower] Refactor MLIR codegen for OpenMP constructs

This patch extracts MLIR codegen logic from various types of OpenMP constructs
and places it into operation-specific functions. This refactoring mainly
targets block constructs and unifying logic for directives that can appear on
their own as well as combined with others.

The processing of clauses that do not apply to the directive being processed is
avoided and code duplication for combined constructs is reduced.

Depends on D156455.

Differential Revision: https://reviews.llvm.org/D156809