[MLIR][OpenMP] NFC: Split OpenMP dialect definitions (#91741)

This patch splits definitions for the OpenMP dialect into multiple files
to simplify the addition of new features, reduce merge conflicts, make
it easier to understand, etc. The split is based on the structure of the
more mature LLVMIR dialect. More specifically:

- The OpenMP dialect definition is located in OpenMPDialect.td.
- Base classes for OpenMP operations and types, as well as generic
OpenMP types are moved to OpenMPOpBase.td.
- OpenMP enumeration attributes, their case attributes and shared base
classes for these are placed in OpenMPEnums.td.
- Other OpenMP attributes are separated into OpenMPAttrDefs.td.
- OpenMPOps.td only contains operation definitions.

Even though this change should be useful on its own, it is intended as a
precursor to a follow-up PR in which operation arguments and attributes
are split into clause-specific classes which are then shared by all
operations to which they apply to. Without this prior change, that
approach would make the OpenMPOps.td file harder to navigate.