Added StringExtractor::DecodeHexU8 && GetHexBytesAvail

DecodeHexU8 returns a decoded hex character pair, returns -1 if a
valid hex pair is not available.

GetHexBytesAvail decodes all available hex pairs.



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@223081 91177308-0d34-0410-b5e6-96231b3b80d8