Rollup merge of #97150 - ChrisDenton:stdio-create_pipe, r=m-ou-se

`Stdio::makes_pipe`

Wrappers around `std::process::Command` may want to be able to override pipe creation. However, [`std::process::Stdio`](https://doc.rust-lang.org/std/process/struct.Stdio.html) is opaque so there's no way to tell if `Command` was told to create new pipes or not.

This is in some ways a more generic (and cross-platform) alternative to #97149. However, unlike that feature, this comes with the price of the user needing to actually create their own pipes rather than reusing the std one. So I think it stands (or not) on its own.

# Example

```rust
#![feature(stdio_makes_pipe)]
use std::process::Stdio;

let io = Stdio::piped();
assert_eq!(io.makes_pipe(), true);
```

library/std/src/process.rs

@@ -1273,6 +1273,22 @@ impl Stdio {
     pub fn null() -> Stdio {
         Stdio(imp::Stdio::Null)
     }
+
+    /// Returns `true` if this requires [`Command`] to create a new pipe.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// #![feature(stdio_makes_pipe)]
+    /// use std::process::Stdio;
+    ///
+    /// let io = Stdio::piped();
+    /// assert_eq!(io.makes_pipe(), true);
+    /// ```
+    #[unstable(feature = "stdio_makes_pipe", issue = "98288")]
+    pub fn makes_pipe(&self) -> bool {
+        matches!(self.0, imp::Stdio::MakePipe)
+    }
 }
 
 impl FromInner<imp::Stdio> for Stdio {