[clang] [clang][analyzer] Support `fflush` in the StreamChecker (PR #74296)

[Balázs Kéri via cfe-commits]

https://github.com/balazske requested changes to this pull request.

For now only the pre-condition is added, the `evalFflush` function is missing.

https://github.com/llvm/llvm-project/pull/74296