[MLIR] XeGPU dialect for Intel GPU - core definitions and base classes (#78483)

This PR follows our previous [RFC
](https://discourse.llvm.org/t/rfc-add-xegpu-dialect-for-intel-gpus/75723

)
to add XeGPU dialect definition for Intel GPUs. It contains dialect,
type, attributes and operators definitions, as well as testcases for
semantic checks. The lowering and optimization passes will be issued
with separated passes.

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Co-authored-by: Mehdi Amini <joker.eph@gmail.com>