when building compiler, error message paths are no longer relative to `x.py`

[nikomatsakis]

It used to be that when you ran

./x.py build

you would get an error message relative to that directory:

error: foo bar
  --> src/librustc/lib.rs

but now I just get --> librustc/lib.rs.

This seems less good; among other things, it throws off emacs =) but I imagine any tooling that is trying to resolve paths and doesn't know about the src directory being special.

Anybody know when that happened?

(I'm not really sure who to cc here...)

[nikomatsakis]

dup of #47355