Safe Transmute: Enable handling references, including recursive types

This patch enables support for references in Safe Transmute, by generating
nested obligations during trait selection. Specifically, when we call
`confirm_transmutability_candidate(...)`, we now recursively traverse the
`rustc_transmute::Answer` tree and create obligations for all the `Answer`
variants, some of which include multiple nested `Answer`s.

Also, to handle recursive types, enable support for coinduction for the Safe
Transmute trait (`BikeshedIntrinsicFrom`) by adding the `#[rustc_coinduction]`
annotation.

Also fix some small logic issues when reducing the `or` and `and` combinations
in `rustc_transmute`, so that we don't end up with additional redundant
`Answer`s in the tree.

Co-authored-by: Jack Wrenn <[email protected]>

Author: bryangarza

core/src/mem/transmutability.rs

@@ -5,6 +5,7 @@
 /// notwithstanding whatever safety checks you have asked the compiler to [`Assume`] are satisfied.
 #[unstable(feature = "transmutability", issue = "99571")]
 #[lang = "transmute_trait"]
+#[rustc_coinductive]
 pub unsafe trait BikeshedIntrinsicFrom<Src, Context, const ASSUME: Assume = { Assume::NOTHING }>
 where
     Src: ?Sized,