Add CastKind::Transmute to MIR

Updates `interpret`, `codegen_ssa`, and `codegen_cranelift` to consume the new cast instead of the intrinsic.

Includes `CastTransmute` for custom MIR building, to be able to test the extra UB.

src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs

@@ -176,6 +176,9 @@ fn check_rvalue<'tcx>(
             // FIXME(dyn-star)
             unimplemented!()
         },
+        Rvalue::Cast(CastKind::Transmute, _, _) => {
+            Err((span, "transmute can attempt to turn pointers into integers, so is unstable in const fn".into()))
+        },
         // binops are fine on integers
         Rvalue::BinaryOp(_, box (lhs, rhs)) | Rvalue::CheckedBinaryOp(_, box (lhs, rhs)) => {
             check_operand(tcx, lhs, span, body)?;

src/tools/miri/tests/fail/never_transmute_humans.rs

@@ -7,6 +7,6 @@ struct Human;
 
 fn main() {
     let _x: ! = unsafe {
-        std::mem::transmute::<Human, !>(Human) //~ ERROR: transmuting to uninhabited
+        std::mem::transmute::<Human, !>(Human) //~ ERROR: entering unreachable code
     };
 }

src/tools/miri/tests/fail/never_transmute_humans.stderr

@@ -1,8 +1,8 @@
-error: Undefined Behavior: transmuting to uninhabited type
+error: Undefined Behavior: entering unreachable code
   --> $DIR/never_transmute_humans.rs:LL:CC
    |
 LL |         std::mem::transmute::<Human, !>(Human)
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ transmuting to uninhabited type
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ entering unreachable code
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

src/tools/miri/tests/fail/never_transmute_void.rs

@@ -10,11 +10,13 @@ mod m {
     pub struct Void(VoidI);
 
     pub fn f(v: Void) -> ! {
-        match v.0 {} //~ ERROR: entering unreachable code
+        match v.0 {}
+        //~^ ERROR: entering unreachable code
     }
 }
 
 fn main() {
     let v = unsafe { std::mem::transmute::<(), m::Void>(()) };
-    m::f(v); //~ NOTE: inside `main`
+    m::f(v);
+    //~^ NOTE: inside `main`
 }