Fix build failure on Linux and BSD by reverting r287597

Linux and BSD builds failing after this changes from rev 287597.

llvm-svn: 287631
diff --git a/lldb/source/Interpreter/CommandInterpreter.cpp b/lldb/source/Interpreter/CommandInterpreter.cpp
index 3665151..251f591 100644
--- a/lldb/source/Interpreter/CommandInterpreter.cpp
+++ b/lldb/source/Interpreter/CommandInterpreter.cpp
@@ -1859,8 +1859,9 @@
     // put an empty string in element 0.
     std::string command_partial_str;
     if (cursor_index >= 0)
-      command_partial_str =
-          parsed_line[cursor_index].ref.take_front(cursor_char_position);
+      command_partial_str.assign(parsed_line.GetArgumentAtIndex(cursor_index),
+                                 parsed_line.GetArgumentAtIndex(cursor_index) +
+                                     cursor_char_position);
 
     std::string common_prefix;
     matches.LongestCommonPrefix(common_prefix);
@@ -1871,7 +1872,7 @@
     // Only do this if the completer told us this was a complete word,
     // however...
     if (num_command_matches == 1 && word_complete) {
-      char quote_char = parsed_line[cursor_index].quote;
+      char quote_char = parsed_line.GetArgumentQuoteCharAtIndex(cursor_index);
       common_prefix =
           Args::EscapeLLDBCommandArgument(common_prefix, quote_char);
       if (quote_char != '\0')