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

[Henry Jiang via llvm-commits]

mustartt wrote:

Yes `-fno-builtin-wcslen` is the recommendation for source/build fix and has been applied to projects like WINE already. 

But there are still a handful of projects where `-fshort-wchar` is used, and this optimization is likely to cause an ABI mismatch for them without providing any meaningful optimization. For those projects, I think we should disable this promotion by default (which is the intent of this patch). But how we want to go about telling `TLI` that the wchar family of lib functions might contain an ABI mismatch and should be disabled needs a bit more discussion to flush out.



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