Clarify warn_cxx98_compat_attribute diagnostic

Various attribute flavours are supported in C++98. Make it clear that this
compatibility warning relates specifically to C++11-style generalized
attributes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199053 91177308-0d34-0410-b5e6-96231b3b80d8