[libc++] Ensure that passing predicates through `reference_wrapper` doesn't kill `desugars_to` optimizations

[ldionne]

We do that in e.g. ranges::contains where we pass a reference_wrapper to the projection to ranges::find. Does that turn off optimizations?

[ldionne]

I just confirmed that this actually breaks our optimizations. Calling std::mismatch with a reference_wrapper prevents it from being optimized properly.