Fix use after free.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@134234 91177308-0d34-0410-b5e6-96231b3b80d8
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);
 }