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/Interpreter/OptionValueLanguage.cpp b/lldb/source/Interpreter/OptionValueLanguage.cpp
index a59419f..2659762 100644
--- a/lldb/source/Interpreter/OptionValueLanguage.cpp
+++ b/lldb/source/Interpreter/OptionValueLanguage.cpp
@@ -61,7 +61,7 @@
         error_strm.Printf("%s%s%s", "    ",
                           Language::GetNameForLanguageType(language), "\n");
       }
-      error.SetErrorString(error_strm.GetData());
+      error.SetErrorString(error_strm.GetString());
     }
   } break;