[clang] [NFC] Refactor `[[nodiscard]]` test to not use macros and run under `-pedantic` (PR #95762)

[Mital Ashok via cfe-commits]

MitalAshok wrote:

CC @AaronBallman @Sirraide

More tests will be added in https://github.com/llvm/llvm-project/pull/95112/files#diff-50c6ce984d17856a8b61d98f625dacec78bfbbe01d2e9fbab80b27c771e3db99R144 but these changes are unrelated enough to warrant a separate patch

https://github.com/llvm/llvm-project/pull/95762