function-args-by-value: Don't warn in copy-ctors
function-args-by-value: Don't warn in copy-ctors

copy-ctors can't take by value. This also fixes the unit-test
which somehow only started complaining with clang 5.0

nyalldawson/clazy,nyalldawson/clazy,nyalldawson/clazy,nyalldawson/clazy