GPU dialect: introduce custom syntax for gpu.launch

    This syntax removes boilerplate and verbose list of region arguments in the
    header of the entry block.  It groups operands into segments related to GPU
    blocks, GPU threads as well as the operands that are forwarded to the kernel.
    The two former segments are also used to give names to the region arguments
    that are used for GPU blocks and threads inside the kernel body region.

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PiperOrigin-RevId: 246792329