Rollup merge of rust-lang#131120 - tgross35:stabilize-const_option, r=RalfJung

Stabilize `const_option`

This makes the following API stable in const contexts:

```rust
impl<T> Option<T> {
    pub const fn as_mut(&mut self) -> Option<&mut T>;
    pub const fn expect(self, msg: &str) -> T;
    pub const fn unwrap(self) -> T;
    pub const unsafe fn unwrap_unchecked(self) -> T;
    pub const fn take(&mut self) -> Option<T>;
    pub const fn replace(&mut self, value: T) -> Option<T>;
}

impl<T> Option<&T> {
    pub const fn copied(self) -> Option<T>
    where T: Copy;
}

impl<T> Option<&mut T> {
    pub const fn copied(self) -> Option<T>
    where T: Copy;
}

impl<T, E> Option<Result<T, E>> {
    pub const fn transpose(self) -> Result<Option<T>, E>
}

impl<T> Option<Option<T>> {
    pub const fn flatten(self) -> Option<T>;
}
```

The following functions make use of the unstable `const_precise_live_drops` feature:

- `expect`
- `unwrap`
- `unwrap_unchecked`
- `transpose`
- `flatten`

Fixes: <rust-lang#67441>

Author: tgross35

tests/ui/doc/doc-fixable.fixed

@@ -3,7 +3,7 @@
 
 #![allow(dead_code, incomplete_features)]
 #![warn(clippy::doc_markdown)]
-#![feature(custom_inner_attributes, generic_const_exprs, const_option)]
+#![feature(custom_inner_attributes, generic_const_exprs)]
 #![rustfmt::skip]
 
 /// The `foo_bar` function does _nothing_. See also `foo::bar`. (note the dot there)

tests/ui/doc/doc-fixable.rs

@@ -3,7 +3,7 @@
 
 #![allow(dead_code, incomplete_features)]
 #![warn(clippy::doc_markdown)]
-#![feature(custom_inner_attributes, generic_const_exprs, const_option)]
+#![feature(custom_inner_attributes, generic_const_exprs)]
 #![rustfmt::skip]
 
 /// The foo_bar function does _nothing_. See also foo::bar. (note the dot there)