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

[Ben Shi via cfe-commits]

benshi001 wrote:

I can not understand why the new test `error_fflush_1` failed on windows, it did succeed on my local linux. So shall we 
1. Add a `#if _win32` in the test?
2. Or only commit `fflush(not_null_stream) == 0` part, and leave `fflush(NULL) == 0` in the next patch?

I hope the second one, at least we can break down this change.

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