Start using pattern types in libcore cc rust-lang/rust#135996 Replaces the innards of `NonNull` with `*const T is !null`. This does affect LLVM's optimizations, as now reading the field preserves the metadata that the field is not null, and transmuting to another type (e.g. just a raw pointer), will also preserve that information for optimizations. This can cause LLVM opts to do more work, but it's not guaranteed to produce better machine code. Once we also remove all uses of rustc_layout_scalar_range_start from rustc itself, we can remove the support for that attribute entirely and handle all such needs via pattern types