[llvm] [TLI][NFC] Mark TargetLibraryInfo::getLibFunc as [[nodiscard]] (PR #215746)

[Kito Cheng via llvm-commits]

kito-cheng wrote:

> This looks fine to me -- but wouldn't it be better to change getLibFunc() to actually return LibFunc? (And LibFunc::NotLibFunc if it's not one.)

That's good suggesion, I was intend to not change the interface, however I am happy to update this PR with this way if it's OK :)

I guess I got the habit from the GCC community about avoiding changing the API as much as possible. :P

https://github.com/llvm/llvm-project/pull/215746