[MERGE #4181 @jianchun] fix incorrect int64 stringify
[MERGE #4181 @jianchun] fix incorrect int64 stringify

Merge pull request #4181 from jianchun:projint64

TaggedInt::SignedToString was casting `int64` to `unsigned long` and losing
higher 4-byte value.

Microsoft/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore