Diagnose GNU-style attributes preceding virt-specifiers, but only when the attribute is known to GCC. Clang accepts attributes in this position, but
GCC does not, so this is a GCC-compat warning. If the attribute is not known to GCC, then the diagnostic is suppressed.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214730 91177308-0d34-0410-b5e6-96231b3b80d8