[Flang][OpenMP][Lower] Refactor lowering of compound constructs

This patch simplifies the lowering from PFT to MLIR of OpenMP compound
constructs (i.e. combined and composite).

The new approach consists of iteratively processing the outermost leaf
construct of the given combined construct until it cannot be split further.
Both leaf constructs and composite ones have `gen...()` functions that are
called when appropriate.

This approach enables treating a leaf construct the same way regardless of if
it appeared as part of a combined construct, and it also enables the lowering
of composite constructs as a single unit.

Previous corner cases are now handled in a more straightforward way and
comments pointing to the relevant spec section are added. Directive sets are
also completed with missing LOOP related constructs.