This commit is a fix for #54408 where on nightly right now whenever
generics are inlined the path name listed for the inlined function's
debuginfo is a relative path to the cwd, which surely doesn't exist!
Previously on beta/stable the debuginfo mentioned an absolute path which
still didn't exist, but more predictably didn't exist.
The change between stable/nightly is that nightly is now compiled with
`--remap-path-prefix` to give a deterministic prefix to all
rustc-generated paths in debuginfo. By using `--remap-path-prefix` the
previous logic would recognize that the cwd was remapped, causing the
original relative path name of the standard library to get emitted. If
`--remap-path-prefix` *wasn't* passed in then the logic would create an
absolute path name and then create a new source file entry.
The fix in this commit is to apply the "recreate the source file entry
with an absolute path" logic a bit more aggresively. If the source
file's name was remapped then we don't touch it, but otherwise we always
take the working dir (which may have been remapped) and then join it to
the file to ensure that we process all relative file names as well.
The end result is that the standard library should have an absolute path
for all file names in debuginfo (using our `--remap-path-prefix`
argument) as it does on stable after this patch.
Closes#54408
Remove experimental -Zremap-path-prefix-from/to, and replace it with
the stabilized --remap-path-prefix=from=to variant.
This is an implementation for issue of #41555.
