[llvm] [Support] FIx a warning (PR #138546)

[Nikita Popov via llvm-commits]

nikic wrote:

@kazutakahirata And you're not using `-Wdocumentation` either?

This is weird because this warning doesn't seem to be part of any warnings the build system enables (like `-Wextra`).

https://github.com/llvm/llvm-project/pull/138546