cm_cxx_features: Ignore Clang unused-argument warnings

Filter out `-Wunused-command-line-argument` warnings from Clang (that
can be caused by user-specified flags) so that they do not break our
checks for C++ feature availability.

This extends commit 71b65abca2 (C++ feature checks: Filter out warnings
caused by user flags, 2017-09-19, v3.10.0-rc1~90^2).