Rollup merge of #90377 - WaffleLapkin:const_slice_from_raw_parts, r=oli-obk

Make `core::slice::from_raw_parts[_mut]` const

Responses to #90012 seem to allow ``@rust-lang/wg-const-eval`` to decide on use of `const_eval_select`, so we can make `core::slice::from_raw_parts[_mut]` const :)

---
This PR marks the following APIs as const:
```rust
// core::slice
pub const unsafe fn from_raw_parts<'a, T>(data: *const T, len: usize) -> &'a [T];
pub const unsafe fn from_raw_parts_mut<'a, T>(data: *mut T, len: usize) -> &'a mut [T];
```
---

Resolves #90011
r? ``@oli-obk``

library/core/src/lib.rs

@@ -105,6 +105,7 @@
 #![feature(const_caller_location)]
 #![feature(const_cell_into_inner)]
 #![feature(const_discriminant)]
+#![cfg_attr(not(bootstrap), feature(const_eval_select))]
 #![feature(const_float_bits_conv)]
 #![feature(const_float_classify)]
 #![feature(const_fmt_arguments_new)]