Rollup merge of #80491 - RalfJung:dangling-of-val, r=oli-obk

Miri: make size/align_of_val work for dangling raw ptrs

This is needed for https://github.com/rust-lang/rust/issues/80365#issuecomment-752128105.

r? `@oli-obk`

compiler/rustc_mir/src/interpret/intrinsics.rs

@@ -141,9 +141,11 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             }
 
             sym::min_align_of_val | sym::size_of_val => {
-                let place = self.deref_operand(args[0])?;
+                // Avoid `deref_operand` -- this is not a deref, the ptr does not have to be
+                // dereferencable!
+                let place = self.ref_to_mplace(self.read_immediate(args[0])?)?;
                 let (size, align) = self
-                    .size_and_align_of(place.meta, place.layout)?
+                    .size_and_align_of_mplace(place)?
                     .ok_or_else(|| err_unsup_format!("`extern type` does not have known layout"))?;
 
                 let result = match intrinsic_name {