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/Go/GoFormatterFunctions.cpp b/lldb/source/Plugins/Language/Go/GoFormatterFunctions.cpp
index 110fc63..0f78f64 100644
--- a/lldb/source/Plugins/Language/Go/GoFormatterFunctions.cpp
+++ b/lldb/source/Plugins/Language/Go/GoFormatterFunctions.cpp
@@ -42,7 +42,7 @@
         lldb::addr_t object_at_idx = m_base_data_address;
         object_at_idx += idx * m_type.GetByteSize(nullptr);
         cached = CreateValueObjectFromAddress(
-            idx_name.GetData(), object_at_idx,
+            idx_name.GetString(), object_at_idx,
             m_backend.GetExecutionContextRef(), m_type);
       }
       return cached;