[clang] [clang] Reject constexpr-unknown values as constant expressions more consistently (PR #129952)

[via cfe-commits]

llvmbot wrote:

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Failed to cherry-pick: 42d49a77241df73a17cb442973702fc460e7fb90

https://github.com/llvm/llvm-project/actions/runs/13755084406

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/129952