Changed the 'newest_LLVM' variable to 3.5 and fixed an error in help message

This commit affects alloy.py performance testing. The --perf_LLVM option will compare the 'latest' stable LLVM (that is 3.5 currently) with the specified one. The 'latest' is changed from 3.4 to 3.5 and the version in help message is changed from 3.3 to 3.5.