Update for llvm api change.

llvm-svn: 244849
diff --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp
index 6c38c80..2f43510 100644
--- a/lld/ELF/Writer.cpp
+++ b/lld/ELF/Writer.cpp
@@ -255,9 +255,10 @@
 }
 
 template <class ELFT> void Writer<ELFT>::openFile(StringRef Path) {
-  std::error_code EC = FileOutputBuffer::create(Path, FileSize, Buffer,
-                                                FileOutputBuffer::F_executable);
-  error(EC, Twine("failed to open ") + Path);
+  ErrorOr<std::unique_ptr<FileOutputBuffer>> BufferOrErr =
+      FileOutputBuffer::create(Path, FileSize, FileOutputBuffer::F_executable);
+  error(BufferOrErr, Twine("failed to open ") + Path);
+  Buffer = std::move(*BufferOrErr);
 }
 
 // Write section contents to a mmap'ed file.