Warn that scoped enumerations are a C++11 extenstion when compiling in
C++98 mode.  This improves on the previous diagnostic message of:

error: expected identifier or '{'


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@180076 91177308-0d34-0410-b5e6-96231b3b80d8