Rollup merge of #139671 - m-ou-se:proc-macro-span, r=dtolnay

Proc macro span API redesign: Replace proc_macro::SourceFile by Span::{file, local_file}

Simplification/redesign of the unstable proc macro span API, tracked in https://github.com/rust-lang/rust/issues/54725:

Before:

```rust
impl Span {
    pub fn line(&self) -> usize;
    pub fn column(&self) -> usize;
    pub fn source_file(&self) -> SourceFile;
}

#[derive(Clone, Debug, PartialEq, Eq)]
pub struct SourceFile { .. }

impl !Send for SourceFile {}
impl !Sync for SourceFile {}

impl SourceFile {
    pub fn path(&self) -> PathBuf;
    pub fn is_real(&self) -> bool;
}
```

After:

```rust
impl Span {
    pub fn line(&self) -> usize;
    pub fn column(&self) -> usize;
    pub fn file(&self) -> String; // Mapped file name, for display purposes.
    pub fn local_file(&self) -> Option<PathBuf>; // Real file name as it exists on disk.
}
```

This resolves the last blocker for stabilizing these methods. (Stabilizing will be a separate PR with FCP.)

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