Automated checking for C++ when determining what argument to send to the diagnostic for attribute subjects. In turn, this allows the Subjects to be enabled for some more attributes and improves diagnostics. Updated a test case based on the improved diagnostic.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195864 91177308-0d34-0410-b5e6-96231b3b80d8