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
(cherry picked from commit 2b8b783)

lldb/tools/debugserver/source/RNBRemote.cpp

@@ -1643,7 +1643,9 @@ rnb_err_t RNBRemote::HandlePacket_qLaunchSuccess(const char *p) {
     return SendPacket("OK");
   std::ostringstream ret_str;
   std::string status_str;
-  ret_str << "E" << m_ctx.LaunchStatusAsString(status_str);
+  std::string error_quoted = binary_encode_string
+               (m_ctx.LaunchStatusAsString(status_str));
+  ret_str << "E" << error_quoted;
 
   return SendPacket(ret_str.str());
 }
@@ -2677,8 +2679,9 @@ std::string cstring_to_asciihex_string(const char *str) {
   std::string hex_str;
   hex_str.reserve (strlen (str) * 2);
   while (str && *str) {
+    uint8_t c = *str++;
     char hexbuf[5];
-    snprintf (hexbuf, sizeof(hexbuf), "%02x", *str++);
+    snprintf (hexbuf, sizeof(hexbuf), "%02x", c);
     hex_str += hexbuf;
   }
   return hex_str;