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/maybe_transmutable/query_context.rs

@@ -5,8 +5,6 @@ pub(crate) trait QueryContext {
     type Def: layout::Def;
     type Ref: layout::Ref;
     type Scope: Copy;
-
-    fn min_align(&self, reference: Self::Ref) -> usize;
 }
 
 #[cfg(test)]
@@ -31,10 +29,6 @@ pub(crate) mod test {
         type Def = Def;
         type Ref = !;
         type Scope = ();
-
-        fn min_align(&self, reference: !) -> usize {
-            unimplemented!()
-        }
     }
 }
 
@@ -48,9 +42,5 @@ mod rustc {
         type Ref = layout::rustc::Ref<'tcx>;
 
         type Scope = Ty<'tcx>;
-
-        fn min_align(&self, reference: Self::Ref) -> usize {
-            unimplemented!()
-        }
     }
 }