Add a setting escape-non-printables that drives whether the StringPrinter...

Add a setting escape-non-printables that drives whether the StringPrinter should or should not escape sequences such as \t, \n, .. and generally any non-printing character

The recent StringPrinter changes made this behavior the default, and the setting defaults to yes
If you want to change this behavior and see non-printables unescaped (e.g. "a\tb" as "a    b"), set it to false

Fixes rdar://12969594

llvm-svn: 221399