A little ArrayRef'ization.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182074 91177308-0d34-0410-b5e6-96231b3b80d8
Use 'c++' not 'cxx' in these diagnostic group names for consistency.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182075 91177308-0d34-0410-b5e6-96231b3b80d8