[llvm] [LoopIdiom] Don't introduce `wcslen` calls when `wchar` size is not the default (PR #213629)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

> Do we have a reason to not optimize explicit wcslen calls with -fshort-wchar?

LibCallSimplifier::optimizeWcslen is very broken if the char width is wrong.

There are a few minor optimizations with attributes that don't require knowing the char width, but probably not worthwhile.

https://github.com/llvm/llvm-project/pull/213629