[lldb] Print unprintable characters as unsigned

When specifying the C-string format for dumping memory, we treat
unprintable characters as signed. Whether a character is signed or not
is implementation defined, but all printable characters are signed.
Therefore it's fair to assume that unprintable characters are unsigned.

Before this patch, "\xcf\xfa\xed\xfe\f" would be printed as
"\xffffffcf\xfffffffa\xffffffed\xfffffffe\f". Now we correctly print the
original string.

rdar://111126134

Differential revision: https://reviews.llvm.org/D153644