Don't lint *_interior_mutable_const on unions due to potential ICE.

Author: Jarcho

clippy_lints/src/non_copy_const.rs

@@ -149,6 +149,9 @@ fn is_value_unfrozen_raw<'tcx>(
             // the fact that we have to dig into every structs to search enums
             // leads us to the point checking `UnsafeCell` directly is the only option.
             ty::Adt(ty_def, ..) if ty_def.is_unsafe_cell() => true,
+            // As of 2022-09-08 miri doesn't track which union field is active so there's no safe way to check the
+            // contained value.
+            ty::Adt(def, ..) if def.is_union() => false,
             ty::Array(..) | ty::Adt(..) | ty::Tuple(..) => {
                 let val = cx.tcx.destructure_mir_constant(cx.param_env, val);
                 val.fields.iter().any(|field| inner(cx, *field))

tests/ui/crashes/ice-9445.rs

@@ -0,0 +1,3 @@
+const UNINIT: core::mem::MaybeUninit<core::cell::Cell<&'static ()>> = core::mem::MaybeUninit::uninit();
+
+fn main() {}