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/Plugins/Language/CPlusPlus/LibCxx.cpp b/lldb/source/Plugins/Language/CPlusPlus/LibCxx.cpp
index a0b2304..82f1a5d 100644
--- a/lldb/source/Plugins/Language/CPlusPlus/LibCxx.cpp
+++ b/lldb/source/Plugins/Language/CPlusPlus/LibCxx.cpp
@@ -153,8 +153,8 @@
   StreamString name;
   name.Printf("[%" PRIu64 "]", (uint64_t)idx);
   ValueObjectSP retval_sp(CreateValueObjectFromData(
-      name.GetData(), DataExtractor(buffer_sp, process_sp->GetByteOrder(),
-                                    process_sp->GetAddressByteSize()),
+      name.GetString(), DataExtractor(buffer_sp, process_sp->GetByteOrder(),
+                                      process_sp->GetAddressByteSize()),
       m_exe_ctx_ref, m_bool_type));
   if (retval_sp)
     m_children[idx] = retval_sp;