\u{00ad} character is sometimes parsed twice

See here: https://godbolt.org/z/sdv7a3

The println! outputs "f\u{ad}\u{ad}cali" - this isn't a println! issue, but the string literal also contains 2 `\u{ad}` characters.

Same example, but a little more visible: the following assert is supposed to pass:
```rust
assert_eq!(6, "f\u{AD}­cali".chars().count());
```

Issue exists since 1.0