Allow virt-specifiers after GNU attributes in member-declarators. GCC doesn't

allow this, and we should warn on it, but it turns out that people were already
relying on this.

We should introduce a -Wgcc-compat warning for this if the attributes are known
to GCC, but we don't currently track enough information about attributes to do
so reliably.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200045 91177308-0d34-0410-b5e6-96231b3b80d8