[clang] [-Wunsafe-buffer-usage] Fix bug in unsafe casts to incomplete types (PR #116433)

[Dana Jansens via cfe-commits]

================
@@ -2270,19 +2270,28 @@ class UnsafeBufferUsageReporter : public UnsafeBufferUsageHandler {
         MsgParam = 5;
       } else if (const auto *ECE = dyn_cast<ExplicitCastExpr>(Operation)) {
         QualType destType = ECE->getType();
+        bool destTypeComplete = true;
+
         if (!isa<PointerType>(destType))
           return;
+        destType = destType.getTypePtr()->getPointeeType();
+        if (const auto *D = destType->getAsTagDecl())
+          destTypeComplete = D->isCompleteDefinition();
 
-        const uint64_t dSize =
-            Ctx.getTypeSize(destType.getTypePtr()->getPointeeType());
+        // If destination type is incomplete, it is unsafe to cast to anyway, no
+        // need to check its' type:
----------------
danakj wrote:

```suggestion
        // need to check its type:
```

https://github.com/llvm/llvm-project/pull/116433