Allow ObjC writeback conversion in cleanup attribute type check (#195318)

Prior to #164440, CheckAssignmentConstraints in the cleanup attribute
handler ran before ObjC lifetime qualifiers were inferred on the
variable. It compared against a type without '__strong' and accepted
both 'T **' to 'T *__autoreleasing *' and 'T **' to 'T
*__unsafe_unretained *'. #164440 reversed the order, so the check now
runs after '__strong' is inferred and rejects both 'T *__strong *' to 'T
*__autoreleasing *' and 'T *__strong *' to
'T *__unsafe_unretained *'.

Fix the valid case by falling back to isObjCWritebackConversion when the
assignment check fails. This re-allows the '__strong' to
'__autoreleasing' writeback conversion while continuing to reject
'__strong' to '__unsafe_unretained'.

rdar://175133715

GitOrigin-RevId: ced50aafb99a09be40385dd35b43f1ff97ac7284