Implement isCXX11FinalKeyword() in terms of isCXX11VirtSpecifier()

It's not worth keeping two copies of the identifier init and comparison code
just to save a pointer coparison.

This should reduce further once we get proper contextual keywords in the token
stream, so having the identifier checks in one place is a step towards that.

Cleanup only.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198814 91177308-0d34-0410-b5e6-96231b3b80d8