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

lldb/source/Expression/LLVMUserExpression.cpp

@@ -153,7 +153,7 @@ LLVMUserExpression::DoExecute(DiagnosticManager &diagnostic_manager,
 
       StreamString ss;
       if (!call_plan_sp || !call_plan_sp->ValidatePlan(&ss)) {
-        diagnostic_manager.PutString(eDiagnosticSeverityError, ss.GetData());
+        diagnostic_manager.PutString(eDiagnosticSeverityError, ss.GetString());
         return lldb::eExpressionSetupError;
       }