Fix use after free.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@134234 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/llvm/MC/SubtargetFeature.h b/include/llvm/MC/SubtargetFeature.h
index ff726ec..fccff03 100644
--- a/include/llvm/MC/SubtargetFeature.h
+++ b/include/llvm/MC/SubtargetFeature.h
@@ -77,7 +77,7 @@
   explicit SubtargetFeatures(const StringRef Initial = "");
 
   /// Features string accessors.
-  StringRef getString() const;
+  std::string getString() const;
 
   /// Adding Features.
   void AddFeature(const StringRef String, bool IsEnabled = true);
diff --git a/lib/MC/SubtargetFeature.cpp b/lib/MC/SubtargetFeature.cpp
index 247f73a..c930baf 100644
--- a/lib/MC/SubtargetFeature.cpp
+++ b/lib/MC/SubtargetFeature.cpp
@@ -182,7 +182,7 @@
 }
 
 
-StringRef SubtargetFeatures::getString() const {
+std::string SubtargetFeatures::getString() const {
   return Join(Features);
 }