Add impl From<!> for Infallible

The reverse conversion unfortunately causes unexpected errors like:

```
error[E0277]: the trait bound `!: std::convert::From<()>` is not satisfied
   --> src/librustc_metadata/encoder.rs:105:9
    |
105 |         self.emit_usize(seq.len)?;
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::convert::From<()>` is not implemented for `!`
    |
    = help: the following implementations were found:
              <! as std::convert::From<std::convert::Infallible>>
    = note: the trait is implemented for `()`. Possibly this error has been caused by changes to Rust's type-inference algorithm (see: rust-lang#48950 for more info). Consider whether you meant to use the type `()` here instead.
    = note: required by `std::convert::From::from`
```

I don’t understand why this error happens.
If I’m reading the code correctly the return types of `emit_usize`
and of the method that contains line 105 are both `Result<(), !>`,
so the expansion of the `?` operator should involve `!: From<!>`,
not `From<()>`.

Is this a type inference bug?

src/libcore/convert.rs

@@ -596,3 +596,10 @@ impl PartialEq for Infallible {
 
 #[stable(feature = "convert_infallible", since = "1.34.0")]
 impl Eq for Infallible {}
+
+#[stable(feature = "convert_infallible", since = "1.34.0")]
+impl From<!> for Infallible {
+    fn from(x: !) -> Self {
+        x
+    }
+}