rustdoc reports diagnostics against `very/../long/PATH.md` instead of normalizing to `PATH.md`

The same repro as #130470 can be used here: https://github.com/fasterthanlime/readme-md-error-reporting

When running cargo +stage1 t, the output is:

---- src/../README.md - (line 5) stdout ----
error: expected `;`, found `}`
 --> src/../README.md:7:16
  |
3 |     let x = 234 // no semicolon here! oh no!
  |                ^ help: add `;` here
4 | }
  | - unexpected token

But I would argue it should be this:

---- README.md - (line 5) stdout ----
error: expected `;`, found `}`
 --> README.md:7:16
  |
3 |     let x = 234 // no semicolon here! oh no!
  |                ^ help: add `;` here
4 | }
  | - unexpected token

I would caution against using canonicalize — which resolves symbolic links etc. and requires the path to exist on disk right now, and rather advise that y'all simply have a state machine working on a stack of path elements, going through:

• []
• ["src"]
• [] // just popped due to ".."
• ["README.md"]

Start with the linked readme-md-error-reporting reproduction and run cargo +stage1 t to observe the rustdoc diagnostic path. Trace where src/../README.md is formatted, then add coverage showing that the diagnostic reports README.md while preserving the reported location and message.