[clang] [Driver] Fix _XOPEN_SOURCE definition on Solaris (PR #137141)

[via cfe-commits]

llvmbot wrote:

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Failed to cherry-pick: `

https://github.com/llvm/llvm-project/actions/runs/14947694098

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/137141