[Sema] Fix -Wcomma in dependent context

When there is a dependent type inside a cast, the CastKind becomes CK_Dependent
instead of CK_ToVoid.  This fix will check that there is a dependent cast,
the original type is dependent, and the target type is void to ignore the cast.

https://bugs.llvm.org/show_bug.cgi?id=39375

llvm-svn: 345111