Allow checking whether a type allows being uninitialized

This is useful for clippy and for the future `MaybeUninit::assume_init`
panics.

compiler/rustc_const_eval/src/util/check_validity_requirement.rs

@@ -30,7 +30,7 @@ pub fn check_validity_requirement<'tcx>(
         return Ok(!layout.abi.is_uninhabited());
     }
 
-    if tcx.sess.opts.unstable_opts.strict_init_checks {
+    if kind == ValidityRequirement::Uninit || tcx.sess.opts.unstable_opts.strict_init_checks {
         might_permit_raw_init_strict(layout, tcx, kind)
     } else {
         let layout_cx = LayoutCx { tcx, param_env: param_env_and_ty.param_env };
@@ -99,6 +99,9 @@ fn might_permit_raw_init_lax<'tcx>(
                 }
                 s.valid_range(cx).contains(val)
             }
+            ValidityRequirement::Uninit => {
+                bug!("ValidityRequirement::Uninit should have been handled above")
+            }
         }
     };