[1.7>master] [MERGE #4035 @rhuanjl] Optimise xplat UtcTimeFromStrCore
[1.7>master] [MERGE #4035 @rhuanjl] Optimise xplat UtcTimeFromStrCore

Merge pull request #4035 from rhuanjl:fastXplatDateFromString

Added a fast path for standard ascii characters to PAL_tolower

Resolves https://github.com/Microsoft/ChakraCore/issues/4008

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