Bail out on “-MF /dev/null”

Bail out on “-MF /dev/null”

This is an alternative fix for #397, based on the observation/assumption
that using “-MF /dev/null” is only ever used as part of a compiler probe
call in combination with “-c /dev/null -o /dev/null”, so there is little
reason to cache the result. The advantage of just bailing out is to
reduce the number of special cases we have to handle.