ELF: Fix a misuse of Twine::toStringRef

While the toStringRef API almost certainly ends up populating the
SmallString here, the correct way to use this API is to use the return
value.

llvm-svn: 284361
diff --git a/lld/ELF/LinkerScript.cpp b/lld/ELF/LinkerScript.cpp
index 8a00e02..54d5a9b 100644
--- a/lld/ELF/LinkerScript.cpp
+++ b/lld/ELF/LinkerScript.cpp
@@ -1018,10 +1018,10 @@
 
 void ScriptParser::addFile(StringRef S) {
   if (IsUnderSysroot && S.startswith("/")) {
-    SmallString<128> Path;
-    (Config->Sysroot + S).toStringRef(Path);
+    SmallString<128> PathData;
+    StringRef Path = (Config->Sysroot + S).toStringRef(PathData);
     if (sys::fs::exists(Path)) {
-      Driver->addFile(Saver.save(Path.str()));
+      Driver->addFile(Saver.save(Path));
       return;
     }
   }