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

[Balázs Kéri via cfe-commits]

================
@@ -299,6 +299,74 @@ void error_fseek_0(void) {
   fclose(F);
 }
 
+void error_fflush_after_fclose(void) {
+  FILE *F = tmpfile();
+  int Ret;
+  fflush(NULL);                      // no-warning
+  if (!F)
+    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}}
+}
+
+void error_fflush_on_open_failed_stream(void) {
+  FILE *F = tmpfile();
+  if (!F) {
+    fflush(F); // no-warning
+    return;
+  }
+  fclose(F);
+}
+
+void error_fflush_on_unknown_stream(FILE *F) {
+  fflush(F);   // no-warning
----------------
balazske wrote:

A `fclose(F)` can be added (after `fflush`) to show that no state-split is made in `fflush`, and no warning appears for NULL stream at the fclose call.

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