[llvm] [IR] Introduce `llvm.strlen.*` intrinsics (PR #135792)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

In general, I'm not really in favor of adding more intrinsics for this sort of thing. TargetLibraryInfo is sort of tricky, but well-understood, and side-stepping it in favor of an intrinsic tends to lead to weird issues with finding symbols in the C library.  I prefer to only use intrinsics when we have to: for constructs with semantics we can't otherwise represent, and constructs that we need to lower to an instruction or short inline sequence.

LoopIdiomRecognize already generates calls to non-intrinsic functions in some cases.

https://github.com/llvm/llvm-project/pull/135792