[ThinLTO] Fix bot failures from r333335

Change value in vector from StringRef to std::string to avoid errors
when trying to initialize from a std::string.

llvm-svn: 333336
diff --git a/llvm/lib/IR/AsmWriter.cpp b/llvm/lib/IR/AsmWriter.cpp
index 7659a6b..1212f8c 100644
--- a/llvm/lib/IR/AsmWriter.cpp
+++ b/llvm/lib/IR/AsmWriter.cpp
@@ -2576,7 +2576,7 @@
 
   // Print module path entries, using the module id as the slot number. To
   // print in order, add paths to a vector indexed by module id.
-  std::vector<std::pair<StringRef, ModuleHash>> moduleVec;
+  std::vector<std::pair<std::string, ModuleHash>> moduleVec;
   std::string RegularLTOModuleName = "[Regular LTO]";
   moduleVec.resize(TheIndex->modulePaths().size());
   for (auto &ModPath : TheIndex->modulePaths()) {