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

[Erich Keane via cfe-commits]

erichkeane wrote:

I've run it as well this way:
```
 /local/home/ekeane/llvm-project/build/bin/clang -cc1 -internal-isystem /local/home/ekeane/llvm-project/build/lib/clang/21/include -nostdsysteminc -verify -Wno-unused -I /local/home/ekeane/llvm-project/clang/test/C/C99/Inputs /local/home/ekeane/llvm-project/clang/test/C/C99/n590.c -fsanitize=address
```

However, I'm not reproducing any problems with that.  @mikaelholmen : Are there any other repro steps we have to do to enable this? 

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