[clang] [clang][analyzer] Support 'fflush' in the StdLibraryFunctionsChecker (PR #76557)

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

Looks good to me, my only nitpick is that perhaps you could use more descriptive test names instead of distinguishing them with `_0` and `_1` suffixes.

Also, perhaps wait a bit (until next year ;) ) before merging this to give other reviewers a chance to look at it.

https://github.com/llvm/llvm-project/pull/76557