[libc-commits] [libc] [libc] Acquire the lock for scanf files (PR #67357)

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michaelrj-google wrote:

I decided to avoid adding complexity by checking if `ungetc_unlocked` is available. It isn't available on either of the systems I usually use to test, so I wasn't sure how I'd test it.

https://github.com/llvm/llvm-project/pull/67357