[clang] [Clang] warn on discarded [[nodiscard]] function results after casting in C (PR #104677)

[via cfe-commits]

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@@ -54,3 +54,9 @@ void test_missiles(void) {
   launch_missiles();
 }
 
+[[nodiscard]] int f3();
+
+void f4() {
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cor3ntin wrote:

```suggestion
void GH104391(void) {
```

https://github.com/llvm/llvm-project/pull/104677