[stdlib] Don’t do full integer conversions in _roundUp
[stdlib] Don’t do full integer conversions in _roundUp

This code is exercised on pretty much every access to ManagedBuffer — let’s keep it as simple as possible.

The overflow case is not expected to ever occur here; this code is used to align offsets within objects, it isn’t expected to get inputs anywhere near Int.max.

roambotics/swift,benlangmuir/swift,benlangmuir/swift,glessard/swift,apple/swift,glessard/swift,apple/swift,roambotics/swift,glessard/swift,glessard/swift,roambotics/swift,benlangmuir/swift,roambotics/swift,benlangmuir/swift,apple/swift,glessard/swift,apple/swift,benlangmuir/swift,apple/swift,roambotics/swift,benlangmuir/swift,apple/swift,glessard/swift,roambotics/swift