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

[David Blaikie via llvm-commits]

https://github.com/dwblaikie approved this pull request.

Looks like OK changes.

I wouldn't mind a unit test for something low level/broadly used like this, but wouldn't insist on it.

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