Auto merge of #140514 - m-ou-se:proc-macro-span-file, r=Amanieu

Stabilize proc_macro::Span::{file, local_file}.

Stabilizes this part of rust-lang/rust#54725:

```rust
impl Span {
    pub fn file(&self) -> String; // Mapped/artificial file name, for display purposes.

    pub fn local_file(&self) -> Option<PathBuf>; // Real file name as it exists on the local file system.
}
```

See also the naming discussion in rust-lang/rust#139903

Author: bors