[MERGE #4518 @obastemur] xplat: retry clang network
[MERGE #4518 @obastemur] xplat: retry clang network

Merge pull request #4518 from obastemur:down_fix

We have tooling build failures on xplat, due to issues with clang source files network. This PR implements retry like tooling around clang source downloader

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