Fix cxx_variable_templates feature check and documentation

This C++ feature has been marked complete since r191549, but the documentation
claimed it wasn't supported at all and the extension check misreported it as
being available in C.

No regression test; this was a short-lived typo.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199292 91177308-0d34-0410-b5e6-96231b3b80d8