[clangd] Attempt to fix failing buildbots after r352494

For failures see:
http://lab.llvm.org:8011/builders/clang-x86_64-linux-abi-test/builds/38501/steps/build-unified-tree/logs/stdio

llvm-svn: 352510

clang-tools-extra/clangd/refactor/Tweak.cpp

@@ -67,7 +67,7 @@ llvm::Expected<std::unique_ptr<Tweak>> prepareTweak(TweakID ID,
   if (!T->prepare(S))
     return llvm::createStringError(llvm::inconvertibleErrorCode(),
                                    "failed to prepare() a check");
-  return T;
+  return std::move(T);
 }
 
 } // namespace clangd