[llvm] [IR][TLI] Cache getLibFunc() result on Function (NFC) (PR #72867)

[Dhruv Chawla via llvm-commits]

https://github.com/dc03 approved this pull request.

LGTM, though it may also be possible to use `NumLibFuncs` as the value for `UnknownLibFunc`.

I don't know enough about TLI to know if there is any way for the caching to break, so I'd also wait for @aeubanks's review.

https://github.com/llvm/llvm-project/pull/72867