Tweak the MCExternalSymbolizer to print references to C string literals
with raw_ostream's write_escaped() method.

For example darwin's otool(1) program that uses the llvm
disassembler now produces disassembly like this:

leaq	0x7b(%rip), %rdi ## literal pool for: "%f\ntoto\n"

and not print the new lines which messes up the output.

rdar://15145300