Removed redundant isxdigit checks and added the ability to GetHexU8() so it...

Removed redundant isxdigit checks and added the ability to GetHexU8() so it can extract an 8 bit hex value if one is available. It will set EOF if "set_eof_on_fail" is true or if out of data. This allows a string decoder to grab a string without losing the last part of the packet with a packet like "414243,abc" (it can extract "ABC" and leave the file position set to the comma).

llvm-svn: 154239