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/API/SBFrame.cpp b/lldb/source/API/SBFrame.cpp
index 31ac0a7..1845b12 100644
--- a/lldb/source/API/SBFrame.cpp
+++ b/lldb/source/API/SBFrame.cpp
@@ -1295,7 +1295,7 @@
               "SBFrame::EvaluateExpression (expr = \"%s\", fetch_dynamic_value "
               "= %u) %s",
               expr, options.GetFetchDynamicValue(),
-              frame_description.GetString().c_str());
+              frame_description.GetData());
         }
 
         exe_results = target->EvaluateExpression(expr, frame, expr_value_sp,