Rollup merge of #92164 - WaffleLapkin:rustc_must_implement_one_of_attr, r=Aaron1011

Implement `#[rustc_must_implement_one_of]` attribute

This PR adds a new attribute — `#[rustc_must_implement_one_of]` that allows changing the "minimal complete definition" of a trait. It's similar to GHC's minimal `{-# MINIMAL #-}` pragma, though `#[rustc_must_implement_one_of]` is weaker atm.

Such attribute was long wanted. It can be, for example, used in `Read` trait to make transitions to recently added `read_buf` easier:
```rust
#[rustc_must_implement_one_of(read, read_buf)]
pub trait Read {
    fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
        let mut buf = ReadBuf::new(buf);
        self.read_buf(&mut buf)?;
        Ok(buf.filled_len())
    }

    fn read_buf(&mut self, buf: &mut ReadBuf<'_>) -> Result<()> {
        default_read_buf(|b| self.read(b), buf)
    }
}

impl Read for Ty0 {}
//^ This will fail to compile even though all `Read` methods have default implementations

// Both of these will compile just fine
impl Read for Ty1 {
    fn read(&mut self, buf: &mut [u8]) -> Result<usize> { /* ... */ }
}
impl Read for Ty2 {
    fn read_buf(&mut self, buf: &mut ReadBuf<'_>) -> Result<()> { /* ... */ }
}
```

For now, this is implemented as an internal attribute to start experimenting on the design of this feature. In the future we may want to extend it:
- Allow arbitrary requirements like `a | (b & c)`
- Allow multiple requirements like
  - ```rust
    #[rustc_must_implement_one_of(a, b)]
    #[rustc_must_implement_one_of(c, d)]
    ```
- Make it appear in rustdoc documentation
- Change the syntax?
- Etc

Eventually, we should make an RFC and make this (or rather similar) attribute public.

---

I'm fairly new to compiler development and not at all sure if the implementation makes sense, but at least it passes tests :)

compiler/rustc_span/src/symbol.rs

@@ -1162,6 +1162,7 @@ symbols! {
         rustc_macro_transparency,
         rustc_main,
         rustc_mir,
+        rustc_must_implement_one_of,
         rustc_nonnull_optimization_guaranteed,
         rustc_object_lifetime_default,
         rustc_on_unimplemented,