Revert "Introduce a string_ostream string builder facilty"

Temporarily back out commits r211749, r211752 and r211754.

llvm-svn: 211814
diff --git a/llvm/lib/CodeGen/MachineScheduler.cpp b/llvm/lib/CodeGen/MachineScheduler.cpp
index f7459bb..0baf2a6 100644
--- a/llvm/lib/CodeGen/MachineScheduler.cpp
+++ b/llvm/lib/CodeGen/MachineScheduler.cpp
@@ -3235,7 +3235,8 @@
   }
 
   static std::string getNodeLabel(const SUnit *SU, const ScheduleDAG *G) {
-    string_ostream SS;
+    std::string Str;
+    raw_string_ostream SS(Str);
     const ScheduleDAGMI *DAG = static_cast<const ScheduleDAGMI*>(G);
     const SchedDFSResult *DFS = DAG->hasVRegLiveness() ?
       static_cast<const ScheduleDAGMILive*>(G)->getDFSResult() : nullptr;