[clang] [clang] Build argument string for clang::warn_unused_result (PR #148090)

[Aaron Ballman via cfe-commits]

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

LGTM aside from a nit with the comments (sorry for causing that confusion in the first place).

https://github.com/llvm/llvm-project/pull/148090