Use UInt32Math in JavascriptString::ToCaseCore
Use UInt32Math in JavascriptString::ToCaseCore

I noticed this while working on the ASCII performance yesterday, and could cause an issue if ICU ever took a string which is at most MaxCharCount and made it more than twice as long, which currently doesn't happen.

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