Partially stabilize const_slice_from_raw_parts

This doesn't stabilize methods working on mutable pointers.

Author: LunaBorowska

example/issue-91827-extern-types.rs

@@ -6,7 +6,6 @@
 // Regression test for issue #91827.
 
 #![feature(const_ptr_offset_from)]
-#![feature(const_slice_from_raw_parts)]
 #![feature(extern_types)]
 
 use std::ptr::addr_of;