Make Unique::as_ptr, NonNull::dangling and NonNull::cast const

Make `Unique::as_ptr` const without feature attribute as it's unstable
Make `NonNull::dangling` and `NonNull::cast` const with `feature = "const_ptr_nonnull"`

src/test/ui/consts/const-ptr-nonnull.rs

@@ -0,0 +1,11 @@
+use std::ptr::NonNull;
+
+fn main() {
+    let x: &'static NonNull<u32> = &(NonNull::dangling());
+    //~^ ERROR borrowed value does not live long enough
+
+    let mut i: i32 = 10;
+    let non_null = NonNull::new(&mut i).unwrap();
+    let x: &'static NonNull<u32> = &(non_null.cast());
+    //~^ ERROR borrowed value does not live long enough
+}

src/test/ui/consts/const-ptr-nonnull.stderr

@@ -0,0 +1,25 @@
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/const-ptr-nonnull.rs:4:37
+   |
+LL |     let x: &'static NonNull<u32> = &(NonNull::dangling());
+   |                                     ^^^^^^^^^^^^^^^^^^^^^ temporary value does not live long enough
+...
+LL | }
+   | - temporary value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/const-ptr-nonnull.rs:9:37
+   |
+LL |     let x: &'static NonNull<u32> = &(non_null.cast());
+   |                                     ^^^^^^^^^^^^^^^^^ temporary value does not live long enough
+LL |     //~^ ERROR borrowed value does not live long enough
+LL | }
+   | - temporary value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0597`.

src/test/ui/consts/const-ptr-unique.rs

@@ -0,0 +1,10 @@
+#![feature(ptr_internals)]
+
+use std::ptr::Unique;
+
+fn main() {
+    let mut i: u32 = 10;
+    let unique = Unique::new(&mut i).unwrap();
+    let x: &'static *mut u32 = &(unique.as_ptr());
+    //~^ ERROR borrowed value does not live long enough
+}

src/test/ui/consts/const-ptr-unique.stderr

@@ -0,0 +1,14 @@
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/const-ptr-unique.rs:8:33
+   |
+LL |     let x: &'static *mut u32 = &(unique.as_ptr());
+   |                                 ^^^^^^^^^^^^^^^^^ temporary value does not live long enough
+LL |     //~^ ERROR borrowed value does not live long enough
+LL | }
+   | - temporary value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0597`.