[llvm] [Diagnostics] Return rvalue reference from temporary argument (PR #127400)

[Jakub Kuderski via llvm-commits]

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@@ -625,14 +626,14 @@ operator<<(RemarkT &R,
 /// Also allow r-value for the remark to allow insertion into a
 /// temporarily-constructed remark.
 template <class RemarkT>
-RemarkT &
+RemarkT &&
 operator<<(RemarkT &&R,
            std::enable_if_t<
                std::is_base_of<DiagnosticInfoOptimizationBase, RemarkT>::value,
                StringRef>
                S) {
   R.insert(S);
-  return R;
+  return std::move(R);
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kuhar wrote:

Then we should be able to handle both cases by dropping the other one and making this use `std::forward`.

https://github.com/llvm/llvm-project/pull/127400