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

[Jonas Hahnfeld via llvm-commits]

hahnjo wrote:

@arsenm thanks for merging. However, for future changes I would prefer to do this myself, both to decide on the author email address (work or private; this merge got it right because I contributed privately outside of work hours) and to make sure I'm available to respond to any issues reported by the bots...

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