[clang] [clang][headers] Including stddef.h always redefines NULL (PR #99727)

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llvmbot wrote:

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Failed to cherry-pick: COMMIT-SHA`

https://github.com/llvm/llvm-project/actions/runs/10063850061

Please manually backport the fix and push it to your github fork.  Once this is done, please create a [pull request](https://github.com/llvm/llvm-project/compare)

https://github.com/llvm/llvm-project/pull/99727