[MERGE #5021 @jackhorton] Add ASCII special case to JavascriptString::ToCaseCore
[MERGE #5021 @jackhorton] Add ASCII special case to JavascriptString::ToCaseCore

Merge pull request #5021 from jackhorton:tocase-ascii

Fixes #5020

This change makes us faster on String.prototype.toLowerCase/toUpperCase than we were before I made any changes to it in the ASCII scenario.

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