[libcxx] Mark do_open, do_get and do_close parameters unused when catopen is missing

When catopen is missing, do_open, do_get and do_close end up being
no-op, and as such their parameters will be unused which triggers a
warning/error when building with -Wunused-parameter.

Differential Revision: https://reviews.llvm.org/D56023

llvm-svn: 351027