Sema: Flexible array members were introduced in C99, diagnose their use in C++

The declaration of a flexible array member was correctly diagnosed as an
extension in C89 mode but not in C++.


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@193918 91177308-0d34-0410-b5e6-96231b3b80d8