Follow-up to r216619: use isCXXCLassMember() instead of trying to
check the context ourselves when selectively allowing late-added
dll attributes on unused free functions and variables (PR20746)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220874 91177308-0d34-0410-b5e6-96231b3b80d8