Fix bug I introduced with assinging a temporary to a StringRef.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@96041 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Basic/Version.cpp b/lib/Basic/Version.cpp
index 0c81fdd..434ddf3 100644
--- a/lib/Basic/Version.cpp
+++ b/lib/Basic/Version.cpp
@@ -55,7 +55,7 @@
   std::string buf;
   llvm::raw_string_ostream OS(buf);
   OS << getClangRepositoryPath();
-  llvm::StringRef Revision = getClangRevision();
+  const std::string &Revision = getClangRevision();
   if (!Revision.empty())
     OS << ' ' << Revision;
   return buf;