Fix a dangling StringRef in FileCollector

FileSpec::GetPath returns a temporary std::string.

llvm-svn: 363770
diff --git a/lldb/source/Utility/FileCollector.cpp b/lldb/source/Utility/FileCollector.cpp
index 566355d..aafa34b 100644
--- a/lldb/source/Utility/FileCollector.cpp
+++ b/lldb/source/Utility/FileCollector.cpp
@@ -134,7 +134,7 @@
 std::error_code FileCollector::WriteMapping(const FileSpec &mapping_file) {
   std::lock_guard<std::mutex> lock(m_mutex);
 
-  llvm::StringRef root = m_overlay_root.GetPath();
+  std::string root = m_overlay_root.GetPath();
 
   m_vfs_writer.setOverlayDir(root);
   m_vfs_writer.setCaseSensitivity(IsCaseSensitivePath(root));