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/Core/Debugger.cpp b/lldb/source/Core/Debugger.cpp
index 1577a38..24715ec 100644
--- a/lldb/source/Core/Debugger.cpp
+++ b/lldb/source/Core/Debugger.cpp
@@ -320,7 +320,7 @@
               stream_sp->Printf("%s\n", error.AsCString());
             }
             if (feedback_stream.GetSize())
-              stream_sp->Printf("%s", feedback_stream.GetData());
+              stream_sp->PutCString(feedback_stream.GetString());
           }
         }
       }