Some updates for 118754. The various places that were checking for 'MISSING' now...

Some updates for 118754.  The various places that were checking for 'MISSING' now call 'isMissing' if they can.  I think this negates some work Adrian did with introducing MissingWithKnownSignature - but we have to make this change for LTW to work properly.  Someone with a bit of time on their hands should go through all the places isMissing() is now called and see if the check should be 'isReallyMissing()' which would allow the MissingWithKnownSignature to get through.  I will raise an enhancement to cover this.