[llvm] [CAS] Close the stderr temp file of the out-of-process validator (PR #226844)

[Steven Wu via llvm-commits]

https://github.com/cachemeifyoucan approved this pull request.

LGTM. I turned on auto merge. Please nominate for release branch after landing. I will land my change immediately after. 

https://github.com/llvm/llvm-project/pull/226844