[clang] [C] Handle comma operator for implicit int->enum conversions (PR #138752)

[Erich Keane via cfe-commits]

================
@@ -12490,6 +12499,17 @@ static void AnalyzeImplicitConversions(
                    (BO->getOpcode() == BO_And ? "&&" : "||"));
         S.Diag(BO->getBeginLoc(), diag::note_cast_operand_to_int);
       }
+    } else if (BO->isCommaOp() && !S.getLangOpts().CPlusPlus) {
+      /// Analyze the given comma operator. The basic idea behind the analysis
+      /// is to analyze the left and right operands slightly differently. The
+      /// left operand needs to check whether the operand itself has an implicit
+      /// conversion, but not whether the left operand induces an implicit
+      /// conversion for the entire comma expression itself. This is similar to
+      /// how CheckConditionalOperand behaves; it's as-if the correct operand
+      /// were directly used for the implicit conversion check.
+      CheckCommaOperand(S, BO->getLHS(), T, BO->getOperatorLoc(), false);
+      CheckCommaOperand(S, BO->getRHS(), T, BO->getOperatorLoc(), true);
----------------
erichkeane wrote:

```suggestion
      CheckCommaOperand(S, BO->getRHS(), T, BO->getOperatorLoc(), /*ExtraCheckForImplicitConversion=*/true);
```

https://github.com/llvm/llvm-project/pull/138752