[Sema] Fix -Wcomma for C89

There is a small difference in the scope flags for C89 versus the other C/C++
dialects.  This change ensures that the -Wcomma warning won't be duplicated or
issued in the wrong location.  Also, the test case is refactored into C and C++
parts, with the C++ parts guarded by a #ifdef to allow the test to run in both
modes.

https://bugs.llvm.org/show_bug.cgi?id=32370

llvm-svn: 345228