Don't allow direct access to StreamString's internal buffer.

This is a large API change that removes the two functions from
StreamString that return a std::string& and a const std::string&,
and instead provide one function which returns a StringRef.

Direct access to the underlying buffer violates the concept of
a "stream" which is intended to provide forward only access,
and makes porting to llvm::raw_ostream more difficult in the
future.

Differential Revision: https://reviews.llvm.org/D26698

llvm-svn: 287152
diff --git a/lldb/source/Commands/CommandObjectMultiword.cpp b/lldb/source/Commands/CommandObjectMultiword.cpp
index d31bba4..8aa2856 100644
--- a/lldb/source/Commands/CommandObjectMultiword.cpp
+++ b/lldb/source/Commands/CommandObjectMultiword.cpp
@@ -260,7 +260,7 @@
     complete_command_name << prefix << " " << command_name;
 
     if (sub_cmd_obj->HelpTextContainsWord(search_word)) {
-      commands_found.AppendString(complete_command_name.GetData());
+      commands_found.AppendString(complete_command_name.GetString());
       commands_help.AppendString(sub_cmd_obj->GetHelp());
     }