rustc needs a way to remap filenames in debug info

[jsgf]

Right now, rustc records its current directory and each literal pathname passed to it in the debug information. This is usually what we want.

However, if rustc is being used in build infrastructure where the actual directory and pathnames differ from the "canonical" ones, it would be useful to be able to remap the pathnames into the canonical form in the debug info, so that they're useful for debugging.

gcc and clang have the -fdebug-prefix-map=old=new option, which matches each source path against the old prefix and remaps it to new in the debug info (non-matching paths are unchanged). I think we can copy this design, with a -C debug-prefix-map=<old>=<new> (though I'm not thrilled that it can't deal with paths with an = in them).

[hanna-kruppe]

Duplicate of #38322

[jsgf]

I searched, but I guess I missed it.