Quote error string from qLaunchSuccess

If the error message from qLaunchSucess included a gdb RSP
metacharacter, it could crash lldb.  Apply the binary
escaping to the string before sending it to lldb; lldb
promiscuously applies the binary escaping protocol on
packets it receives.

Also fix a small bug in cstring_to_asciihex_string where
a high bit character (eg utf-8 chars) would not be
quoted correctly due to signed char fun.

Differential Revision: https://reviews.llvm.org/D79614

rdar://problem/62873581