add option for literal formatting to APInt::toString()

toString() now takes an optional bool argument that,
depending on the radix, adds the appropriate prefix
to the integer's string representation that makes it into a
meaningful C literal, e.g.:

hexademical: '-f' becomes '-0xf'
octal: '77' becomes '077'
binary: '110' becomes '0b110'

Patch by nobled@dreamwidth.org!

llvm-svn: 133032