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

[Balázs Kéri via cfe-commits]

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@@ -299,6 +299,21 @@ void error_fseek_0(void) {
   fclose(F);
 }
 
+void error_fflush(void) {
+  FILE *F = tmpfile();
+  int Ret;
+  fflush(NULL);                      // no-warning
+  if (!F) {
+    if ((Ret = fflush(F)) != EOF)    // no-warning
+      clang_analyzer_eval(Ret == 0); // expected-warning {{TRUE}}
+    return;
+  }
+  if ((Ret = fflush(F)) != 0)
+    clang_analyzer_eval(Ret == EOF); // expected-warning {{TRUE}}
+  fclose(F);
+  fflush(F);                         // expected-warning {{Stream might be already closed}}
+}
+
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balazske wrote:

If the stream error state is reset by `fflush` another test is needed to check if the reset works.

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