Never regard macro rules with compile_error! invocations as unused

Never regard macro rules with compile_error! invocations as unused

The very point of compile_error! is to never be reached, and one of
the use cases of the macro, currently also listed as examples in the
documentation of compile_error, is to create nicer errors for wrong
macro invocations. Thus, we shuuld never warn about unused macro arms
that contain invocations of compile_error.