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

[Balázs Kéri via cfe-commits]

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@@ -299,6 +299,15 @@ void error_fseek_0(void) {
   fclose(F);
 }
 
+void error_fflush(void) {
+  FILE *F = tmpfile();
+  if (!F)
+    return;
+  fclose(F);
+  fflush(F);    // expected-warning {{Stream might be already closed}}
+  fflush(NULL); // no-warning
+}
+
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balazske wrote:

A new test is needed when the passed stream is NULL but not a null constant.

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