Compute transmutability from rustc_target::abi::Layout

In its first step of computing transmutability, `rustc_transmutability`
constructs a byte-level representation of type layout (`Tree`). Previously, this
representation was computed for ADTs by inspecting the ADT definition and
performing our own layout computations. This process was error-prone, verbose,
and limited our ability to analyze many types (particularly default-repr types).

In this PR, we instead construct `Tree`s from `rustc_target::abi::Layout`s. This
helps ensure that layout optimizations are reflected our analyses, and increases
the kinds of types we can now analyze, including:
- default repr ADTs
- transparent unions
- `UnsafeCell`-containing types

Overall, this PR expands the expressvity of `rustc_transmutability` to be much
closer to the transmutability analysis performed by miri. Future PRs will work
to close the remaining gaps (e.g., support for `Box`, raw pointers, `NonZero*`,
coroutines, etc.).

compiler/rustc_transmute/src/lib.rs

@@ -1,6 +1,6 @@
 #![feature(alloc_layout_extra)]
 #![feature(never_type)]
-#![allow(dead_code, unused_variables)]
+#![allow(unused_variables)]
 
 #[macro_use]
 extern crate tracing;
@@ -49,6 +49,8 @@ pub enum Reason<T> {
     DstIsNotYetSupported,
     /// The layout of the destination type is bit-incompatible with the source type.
     DstIsBitIncompatible,
+    /// The destination type is uninhabited.
+    DstUninhabited,
     /// The destination type may carry safety invariants.
     DstMayHaveSafetyInvariants,
     /// `Dst` is larger than `Src`, and the excess bytes were not exclusively uninitialized.