Rollup merge of #137043 - Sky9x:unsafe-pinned-pt1-libs, r=tgross35,RalfJung,WaffleLapkin

Initial `UnsafePinned` implementation [Part 1: Libs]

Initial libs changes necessary to unblock further work on [RFC 3467](https://rust-lang.github.io/rfcs/3467-unsafe-pinned.html).
Tracking issue: #125735

This PR is split off from #136964, and includes just the libs changes:
- `UnsafePinned` struct
- private `UnsafeUnpin` structural auto trait
- Lang items for both
- Compiler changes necessary to block niches on `UnsafePinned`

This PR does not change codegen, miri, the existing `!Unpin` hack, or anything else. That work is to be split into later PRs.

---

cc ``@RalfJung`` ``@Noratrieb``

``@rustbot`` label F-unsafe_pinned T-libs-api

tests/ui/rfcs/rfc-3467-unsafe-pinned/unsafe-pinned-hides-niche.rs

@@ -0,0 +1,29 @@
+//@ check-pass
+// this test ensures that UnsafePinned hides the niche of its inner type, just like UnsafeCell does
+
+#![crate_type = "lib"]
+#![feature(unsafe_pinned)]
+
+use std::num::NonZero;
+use std::pin::UnsafePinned;
+
+macro_rules! assert_size_is {
+    ($ty:ty = $size:expr) => {
+        const _: () = assert!(size_of::<$ty>() == $size);
+    };
+}
+
+assert_size_is!(UnsafePinned<()> = 0);
+assert_size_is!(UnsafePinned<u8> = 1);
+
+assert_size_is!(       UnsafePinned<               u32>    = 4);
+assert_size_is!(       UnsafePinned<       NonZero<u32>>   = 4);
+assert_size_is!(       UnsafePinned<Option<NonZero<u32>>>  = 4);
+assert_size_is!(Option<UnsafePinned<               u32>>   = 8);
+assert_size_is!(Option<UnsafePinned<       NonZero<u32>>>  = 8);
+assert_size_is!(Option<UnsafePinned<Option<NonZero<u32>>>> = 8);
+
+assert_size_is!(       UnsafePinned<       &()>   = size_of::<usize>());
+assert_size_is!(       UnsafePinned<Option<&()>>  = size_of::<usize>());
+assert_size_is!(Option<UnsafePinned<       &()>>  = size_of::<usize>() * 2);
+assert_size_is!(Option<UnsafePinned<Option<&()>>> = size_of::<usize>() * 2);