CTFE: support assert_zero_valid and assert_uninit_valid

Author: RalfJung

core/src/intrinsics.rs

@@ -860,12 +860,14 @@ extern "rust-intrinsic" {
     /// zero-initialization: This will statically either panic, or do nothing.
     ///
     /// This intrinsic does not have a stable counterpart.
+    #[rustc_const_unstable(feature = "const_assert_type2", issue = "none")]
     pub fn assert_zero_valid<T>();
 
     /// A guard for unsafe functions that cannot ever be executed if `T` has invalid
     /// bit patterns: This will statically either panic, or do nothing.
     ///
     /// This intrinsic does not have a stable counterpart.
+    #[rustc_const_unstable(feature = "const_assert_type2", issue = "none")]
     pub fn assert_uninit_valid<T>();
 
     /// Gets a reference to a static `Location` indicating where it was called.