[llvm] [ADT] Remove unused DenseMapInfo::getEmptyKey (PR #201998)

[Fangrui Song via llvm-commits]

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MaskRay wrote:

This is a weak hash function. We should fix it similar to the `DenseMapInfo<T*>` https://github.com/llvm/llvm-project/pull/197390

https://github.com/llvm/llvm-project/pull/201998