[LLDB] Flush stream at the end of PrintCommandOutput

[zequanwu]

Summary

On Windows, lldb doesn't print any error message until exit. This fixes it.

[JDevlieghere]

LGTM

lldb/source/Interpreter/CommandInterpreter.cpp

while (size > 0 && !WasInterrupted()) {
     chunk_size = stream.Write(data, chunk_size);
     lldbassert(size >= chunk_size);
     data += chunk_size;
     size -= chunk_size;
   }
   if (size > 0) {
     stream.Printf("\n... Interrupted.\n");
   }
+  stream.Flush();
 }
 
 bool CommandInterpreter::EchoCommandNonInteractive(
     llvm::StringRef line, const Flags &io_handler_flags) const {
   if (!io_handler_flags.Test(eHandleCommandFlagEchoCommand))
     return false;
 
   llvm::StringRef command = line.trim();