[lldb][NFCI] Rewrite error-handling code in ProcessGDBRemote::MonitorDebugserverProcess

[bulbazord]

Summary

I'm rewriting this for 2 reasons:

1. Although a 1024 char buffer is probably enough space, the error string may grow beyond that and we could lose some information. Using StringStream (as we do in the rest of ProcessGDBRemote) seems like a sensible solution.
2. I am planning on changing UnixSignals::GetSignalAsCString, rewriting things with llvm::formatv will make that process easier.

[JDevlieghere]

LGTM

lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp

3387–3393

lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp

) {
   std::this_thread::sleep_for(std::chrono::milliseconds(500));
 
   // If our process hasn't yet exited, debugserver might have died. If the
   // process did exit, then we are reaping it.
   const StateType state = process_sp->GetState();
 
   if (state != eStateInvalid && state != eStateUnloaded &&
       state != eStateExited && state != eStateDetached) {
-    char error_str[1024];
-    if (signo) {
-      const char *signal_cstr =
+    StreamString stream;
+    if (signo == 0)
+      stream.Format(DEBUGSERVER_BASENAME " died with an exit status of {0:x8}",
+                    exit_status);
+    else {
+      const char *signal_name =
           process_sp->GetUnixSignals()->GetSignalAsCString(signo);
-      if (signal_cstr)
-        ::snprintf(error_str, sizeof(error_str),
-                   DEBUGSERVER_BASENAME " died with signal %s", signal_cstr);
+      const char *format_str = DEBUGSERVER_BASENAME " died with signal {0}";
+      if (signal_name)
+        stream.Format(format_str, signal_name);
       else
-        ::snprintf(error_str, sizeof(error_str),
-                   DEBUGSERVER_BASENAME " died with signal %i", signo);
-    } else {
-      ::snprintf(error_str, sizeof(error_str),
-                 DEBUGSERVER_BASENAME " died with an exit status of 0x%8.8x",
-                 exit_status);
+        stream.Format(format_str, signo);
     }
-
-    process_sp->SetExitStatus(-1, error_str);
+    process_sp->SetExitStatus(-1, stream.GetString());
   }
   // Debugserver has exited we need to let our ProcessGDBRemote know that it no
   // longer has a debugserver instance
   process_sp->m_debugserver_pid = LLDB_INVALID_PROCESS_ID;
 }
 
 void ProcessGDBRemote::KillDebugserverProcess() {
   m_gdb_comm.Disconnect();