Teach raw_ostream to support hex formatting without a prefix '0x'.

Previously using format_hex() would always print a 0x prior to the
hex characters.  This allows this to be optional, so that one can
choose to print (e.g.) 255 as either 0xFF or just FF.

Differential Revision: http://reviews.llvm.org/D7151

llvm-svn: 227108