Rollup merge of #69122 - dtolnay:backtrace, r=cramertj
Rollup merge of #69122 - dtolnay:backtrace, r=cramertj

Backtrace Debug tweaks

- Change Debug representation of disabled and unsupported backtraces to use \<placeholder\> style, same as what we do for debug printing locked mutexes and mutably borrowed refcells;

    ```diff
    - Error { msg: "...", backtrace: disabled backtrace }
    + Error { msg: "...", backtrace: <disabled> }
    ```

- Remove quotes around unresolved symbol names;

    ```diff
    - Backtrace [{ fn: "<unknown>" }]
    + Backtrace [{ fn: <unknown> }]
    ```

- Add quotes around file paths;

    ```diff
    - Backtrace [{ fn: "krate::main", file: /path/to/main.rs, line: 10 }]
    + Backtrace [{ fn: "krate::main", file: "/path/to/main.rs", line: 10 }]
    ```

- Add test.

graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust