Add the new Args / entry-access API.

The long-term goal here is to get rid of the functions
GetArgumentAtIndex() and GetQuoteCharAtIndex(), instead
replacing them with operator based access and range-based for
enumeration.  There are a lot of callsites, though, so the
changes will be done incrementally, starting with this one.

Differential Revision: https://reviews.llvm.org/D26883

llvm-svn: 287597

lldb/source/Commands/CommandObjectFrame.cpp

@@ -473,12 +473,12 @@ public:
                                bool &word_complete,
                                StringList &matches) override {
     // Arguments are the standard source file completer.
-    std::string completion_str(input.GetArgumentAtIndex(cursor_index));
-    completion_str.erase(cursor_char_position);
+    auto completion_str = input[cursor_index].ref;
+    completion_str = completion_str.take_front(cursor_char_position);
 
     CommandCompletions::InvokeCommonCompletionCallbacks(
         GetCommandInterpreter(), CommandCompletions::eVariablePathCompletion,
-        completion_str.c_str(), match_start_point, max_return_elements, nullptr,
+        completion_str, match_start_point, max_return_elements, nullptr,
         word_complete, matches);
     return matches.GetSize();
   }