[MLIR][OpenMP] Normalize handling of entry block arguments (#109808)

This patch introduces a new MLIR interface for the OpenMP dialect aimed
at providing a uniform way of verifying and handling entry block
arguments defined by OpenMP clauses.

The approach consists in defining a set of overrideable methods that
return the number of block arguments the operation holds regarding each
of the clauses that may define them. These by default return 0, but they
are overriden by the corresponding clause through the
`extraClassDeclaration` mechanism.

Another set of interface methods to get the actual lists of block
arguments is defined, which is implemented based on the previously
described methods. These implicitly define a standardized ordering
between the list of block arguments associated to each clause, based on
the alphabetical ordering of their names. They should be the preferred
way of matching operation arguments and entry block arguments to that
operation's first region.

Some updates are made to the printing/parsing of `omp.parallel` to
follow the expected order between `private` and `reduction` clauses, as
well as the MLIR to LLVM IR translation pass to access block arguments
using the new interface. Unit tests of operations impacted by additional
verification checks and sorting of entry block arguments.