[spirv] Add ModuleOp

    This op defines a SPIR-V module using a MLIR region. The region contains
    one block. Module-level operations, including functions definitions,
    are all placed in this block.

    This CL extracts common definitions from SPIRVOps.td into SPIRVBase.td.
    The new op is placed in SPIRVStructureOps.td.

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PiperOrigin-RevId: 250522320